AMS Performance posted up a dynochart comparing a car with mods and stock downpipes to an AMS cast downpipe. They had some pretty impressive gains in spool up time, and horsepower.
Mods to the car were :
AMS Mid pipe and exhaust
STOCK DOWNPIPES WITH CATS
Cobb Access port with an AMS/Forged Performance tune
587 whp and 555 ft lbs of torque
Click though for Cast downpipes dynochart after the jump.
The turbo's are spooling 500 rpm quicker and the mid range is just INSANE!!! Previously at 3500 rpm we were seeing about 10 psi of boost. With the downpipes we are now seeing 20! up top you can see a 25 - 25 whp gain just about EVERYWHERE!
Source : Nagtroc Forums