Personally I've never checked the rpm's with stock verses the jcosta , to tell you the truth I really don't care .
I've seen no difference top end , but a huge difference on take off . Also I burn a little more gasoline , but again
I don't care about using a little more fuel . I am happy for the trade off because the performance is that much better .
The only bad thing I've ever heard about the jcosta , is from people who are to cheap to buy one .