I am a little late to the party. It took a few minutes to read everything but now i'm caught up.
To the OP... I have a Redline with a cold air intake, high flow cat, exhaust, and trifecta budget tune as well as some suspension mods. I would guess its around 280rwhp. I had the tune when I bought the car and I added all of the other mods. What I noticed was that the exhaust changed the sound and did nothing for performance. The cold air intake changed the sound of the intake so you can hear the blow off and also made the exhaust a tiny bit louder. I like the sound others don't. I saw no performance upgrade. I also think it is messing with the tune. The high flow cat made the exhaust a little louder and gave it a better note where I can hear the backpressure as it idles down from 2000 rpms. I noticed no performance upgrade at first and got engine codes. Then one day it stopped throwing codes and the car felt smoother and a bit quicker. Hardly what I would call a performance upgrade though.
Now my goal is the same as yours. Beat Mustangs and Camaros. I can tell you from first hand experience we can handle the V6's. I had some fun with a guy with an acclaimed 350hp v6 mustang and my car was clearly faster. I let off at 90. Now on to the V8s. As my car sits it CANNOT beat the V8s. I have driven them and "raced" them. (not actual street racing just traffic light sprints) The Mustang GT feels faster and is smoother than my Sky, but I like how the boost comes on in the sky. My car pulls harder in top gear too. Not that it has a higher top end, just that when i'm cruising at 80 and floor it in top gear it feels faster than doing the same in the mustang. The Camaro SS powerband isn't as smooth as the mustang and takes longer to get going. I would guess the Mustang is faster. When I raced the mustang... I jumped out in front and was ahead of him, at most a couple car lengths, until midway through third and 70 mph. Then the Mustang passed and kept going. The Camaro was the same story but much closer as I was a few feet behind his rear bumper when we stopped around 90. Had we continued he would have kept pulling away. So NO with basic mods you cannot beat the V8s. But you can scare them for a few seconds when you jump out in front in the beginning.
Now my review of suspension mods. DO ALL OF THEM!!! The stock suspension is bouncy and unsettled. I did the backbone first and it makes the car feel less cheap and a bit stiffer. Just buy the mid grade brace. It is just fine. The probeam got rid of a annoying behavior in the rear where the back end liked to wander like a live axle under hard cornering. By far the best mod from my experience was the BC coilovers. It ELIMINATED the body roll...At least when it is turned up. When it is set to full soft its dare I say comfortable. a little more roll but still way better than stock. And even at full soft it handles better. When it is turned up it is less comfortable. Ok its jarring over rough road. But I only turn it up when I'm going up a mountain and don't care about the ride comfort. All other times it is set to soft. Also invest in tires. I have Michelin Pilot Super Sports and damn do they grip. I have driven my Sky and Camaros and mustangs and a corvette up a mountain and my car out gripped them all. Noticeably so. So get some good tires. It helps the car do what it does best.
A bit wordy, i'm sorry but I hope this helps