When you say 'heat soak', how do you mean? Cooling water getting to hot or ?
Coolant temperature generally stays normal, but the intake, turbo, intercooler, etc. all will start to heat up when you are continuously running 23 PSI, especially if you're not really moving too quickly, that is, driving on the Autobahn going 200+ to get air moving through. If you do this long enough then you'll notice the power fall off, but once the car cools back down it's back to normal.
Mstrjon32 is spot on about heat soak but I wanted to help clarify what it is and why it's bad.
As you know, engines are designed to run with some heat in them...hence the age old adage about allowing an engine to "warm up" before getting on it. You want some heat in the engine. This heat comes from specific parts of the engine though, the head, the combustion chamber of the cylinder, and in the case of force induction cars, the air being compressed to feed the engine.
Your coolant is there to manage the heat produced by the cylinder head and combustion chamber. The effectiveness of that system is controlled by the system's design (flow rate, radiator size, and thermostat) and your tune (timing, boost, when the fans start, when they speed up, and if/how long they stay on after shut down).
The heat from the turbo however, is managed completely differently. There are a couple of things to know as a foundation to understand the concept of heat soak and why it's bad so we'll start there. As most know, air density (how much air you have in a given space) drops as the temperature of that air increases. Thus say a box 1' square contains air at sea level pressure (apx 14.7 psi...but we call that 0 air pressure and below that is vacuum measured in negative numbers while above it is boost measured in positive numbers) and air is allowed to come in or go out. If the temperature inside that box is 60 degrees, it will have more air in it than if the temperature inside that box is 160 degrees.
Making power out of an engine is simply a matter of burning fuel. The more fuel you burn, the more power you make. However, to burn anything you need oxygen and in cars, obviously, we get that from the air around it. Too much though and the gas won't burn...to little and the gas won't burn...the mixture of air and fuel, expressed as a ration, must be right for the gas to burn. Thus if you want to make more power by adding more fuel you have to have more air in the appropriate ratio. That ratio is between 10.0:1 and 18:1 though generally 14.7:1 is considered ideal and anything above that is considered lean. For power, we often run rich in the 11:1-13:1 area.
So here is where the turbo comes in. The turbo is designed to PUSH more air into the engine than it would normally have allowing us to inject more fuel and thus make more power. The problem is, the more you compress a gas, the higher its temperature becomes and thus, as mentioned above, the less dense the air is. Turbo's have what are called "efficiency maps" and these maps tell you how much air the turbo can boost before the amount of heat generated by compressing the air causes the resulting air charge to have less air in it than an air charge with a lower boost pressure may have.
Let me simplify this last part. Say your turbo is designed to run on a 2.0 Liter engine and run up to 23 psi of boost. Sound familiar? Now, you want to make more power so you turn up the boost to say 30 psi. Surprisingly, the car feels much slower. This is because that amount of compression is off the turbo's efficiency map. at 30 psi, the heat generated by compressing the air that much has lowered the air density of the resulting air charge so great, that you are actually sending less air to the engine than you would if you were to run it at only 23 psi. (This is an INCREADILY oversimplification of all the factors that go into how a turbo works but this is a discussion on heat soak.)
Even so, the air leaving that turbo at 23 psi is still MUCH hotter than the air coming into the turbo from the outside world so if you can cool it back down to something close to ambient, you'll have more air going into the engine and thus can inject more fuel and thus make more power. This is what you're intercooler is for.
As the hot air leaves the turbo, it passes through the intercooler before going onto the engine. As it does so, heat passes from the hot air from the turbo into the metal parts of the intercooler. The intercooler then transfers that heat from it's metal bits to the cool air passing over it as you drive down the road. A properly set up turbo/intercooler system should see the temperature of the boosted air leaving it and going into the engine only 1-3 degrees hotter than the temperature of the air going into the turbo.
Now the problem we have is NORMALLY we accelerate for only a few moments then cruise for a long time. This works fine for our intercooler. However, lets say we accelerate hard, then stop, accelerate hard, and stop, and repeat this over and over. Well the intercooler needs air moving over it to cool off and if it's not getting those "cruise" moments where the air coming into it is not too far off the temp of the air outside, it can't cool off. If this is repeated long enough, the intercooler will get hotter and hotter and thus be able to remove less and less heat from the compressed air the turbo feeds it. This can also occur if you are always in boost like when you take a hard drive up a mountain road (though takes longer than the above mentioned process since you ARE moving just not at high speeds) since the intercooler is constantly trying to remove heat. The more heat builds up like this. The longer it takes to cool back down.
This build up of heat over time is what we call heat soak. Now, it doesn't only happen because of the above mentioned conditions. Drag racers try and run their cars on start up only long enough to get them up to temperature and nothing more. Any longer, and the heat from the engine starts to get into their intercoolers and make them less effective. If you make a pass, come back, make a pass, come back, and make another pass...by that third pass you're going to be suffering from heat soak to have it be noticeable and affect your times considerably. On REALLY high horsepower cars, it may affect even the second pass.
Now other things can be heat soaked too and the one that comes to mind in our cars is the Power Steering system. The Power Steering Reservoir relocation modification is all about moving the power steering fluid tank further away from the turbo so the fluid doesn't get heat soaked and overheat by the heat coming off the turbo. In Autocross on a hot day, by the third lap I would experience power steering whine. On the fourth....if we got one...I had a very heavy steering wheel and the power steering would wine the whole lap. Overheating problems such as these are a rapid version of heat soak since if I let the car sit or drove it normally for a while this condition would go away.