youll find that going from an rb30et to an rb26 in terms of response will be a big step down even if the 26 is stock turboed. now, planning to put on 25/30s will make it a even more laggy and a huge shock. thats ok if you dont mind the lag, but if you plan to drive the car mainly on the street then you may be a little disappointed. if you like that low-mid torque of the rb30, then stick with the GT-SS turbos as they offer similar spool to the stockies.
i know i noticed a big response difference with a stock turboed 32 GTR after driving a mildy modified 30et and stock rb25 for the last few years.