Hypergear can provide you with what you want, from what i've seen all the housings etc he has are brand new, not reconditioned
but Ash is spot on the money, the turbo you have is better matched to a 3 - 4 Litre engine, not a 2.5L
and i understand you may not have the funds, but trying to make the turbo you have more responsive is not only a waste of time and money, but you'll find it won't really be what you're looking for, you'll basically be restricting it with such a small rear housing and have fairly erratic power output
a garrett gt3071r or gt3076r 0.82 will both be much better suited to an rb25, in regards to power and response as a package... you won't be dissapointed
the new Garrett GTX3071r 0.82 (the 5 bolt rear garrett design, not modified copies) with correct supporting mods will see power figures of 270-300rwkw on 98ron and response of fullbost (17-19psi) by around 3600-3700rpm ... with slight modifications to the standard heatshields it can be installed to look fairly standard too!... as opposed to your externally gated 3540 which wouldn't look so standard
food for thought