O.K. lets me see if i can explaine this. I have a 2007 gxp. At about 6000 miles i put on amagnaflow 3" exhaust and had the car dynoed. The car came in at 216h.p. I thought it was a little low but thats another story. I few days later i did a 0to60 and a 1/4 mile run on my dash-hawk. 0to 60was 4.9 and 1/4 mile times were in the 13.7,around 98mph. I did this numerous times that day and they were all pretty consistant. Later that year i took off the exhaust,and put the stock exhaust back on. Well now this year i did the gmpp tune and tried the dash-hawk test again. The numbers were exactily the same. The car is stock except for the tune,and 3000 more miles on the speedo. I should have tried the test after taking off the 3" exhaust but i didnt. Maybe the tune compinsated for taking off the exhaust. The reason i took off the magnaflow was it developed a real annoying drone at about 2000rpm.
Oh yeah the car is a auto.
If you gotta ask "how much?". you cant afford it.