Well it's not necessairly the gains, Steve is right with the 20 range, it could even be a bit higher, but you should see that gain all across the curve which is important.
Also, what you feel more than anything else is the reduced torque management. It feels like you pick up 100 because you finally get all that you originally paid for
Break them loose at 30!

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2006 Chrysler 300C SRT8
- stock with AFE II intake
12.94@109.1 2.00 60
- now Magnaflow HF cats, CORSA and Predator with CMR tune by Ken Bjonnes at DCX Depot...and a 185 t-stat & BT catch can
Bests(all done on Toyo 255s):
60 - 1.939...1/8 - 8.14...1/4 - 12.54...mph 114.1