yeh i dunno about this personally i think rb20's are highly underrated and very usable up to the 250kw range, i'd never remove a running one for a 25.
when my last rb20 blew a headgasket i went engine hunting, i already had a rb25 box in the car so i got a rb25 engine cheap and dropped that in.
now both of these engines were running stock turbos with 1 bar of boost the rb20 would be around 160kw and the 25 was making 193kw.
the 25 was a decent upgrade but if i had a 25 turbo on the 20 i dont think the gap would of been all that great.....
i also think its crazy to rebuild a perfectly good running engine but if your deadset on doing a rebuild i'd be looking at a 25/30 build for the 3L's of fury!