dude,
the TD06-25G is sold as part of the RB25DET kit as it is a larger turbo than the TD06-20G, which is suited to the larger RB25 (2.5L) engine.
although, it is known that exhaust manifold stud patterns are the same b/w the RB20 & RB25 engines so buying a TD06-25G kit should do u fine, but not on the standard 2L RB20DET engine. It will be very laggy and UNresponsive.
i actually toyed with this idea as i am purchasing an R32 GTS-T which had an RB24DET in it with the TD06-25G kit, although the engine was damaged and a standard RB20DET is going in there with a Garrett T3 turbo, until i can rebuild the engine again
and before you ask, NO, i'm not really planning on selling the turbo kit at the moment.