Hey man

Not too sure about the hpower gain but yes you would feel a diferance
due to higher compression , but it also depends if you are using USDM
or JDM ITR pistons USDM typeR pistons are a little higher compression than a USDM GSR so u wouldnt feel too much.
Though if you go with JDM TypeR you will be going from 10.1 too 11.1
compression = a nice little gain (:

I am about to do the sam thing in my GSR
but i am getting larger pistons
81.25 over bore 11.1 compression
but i will be doing the whole motor with cams and ect to complement
the higher compression pistons !

If you need any aftermarket parts
Let me know I own a business

CONTACT INFO:
Greg
813-641-1471
813-846-0853