I had a GT35R .82 top mount on my standard RB25 (with poncams) and it was a bit laggy, but still a lot of fun...kicked hard on boost.
The setup is pretty similar to what I had...ext. gate, bosch 044, top mount, 850cc inj (no front facing intake plenum though)
You definitely wont get into the max efficiency range of the turbo with a standard 25 bottom end, but switching that head over to the 30 bottom end later will sort that out.