I wouldn't bother with the DP unless you are going the whole hog i.e. cams, programmable injection, forgies etc. But to answer your question full boost should around the 4000rpm mark give or take 200rpm. This turbo was designed for the SR20DET. I'm not saying don't do it but unless you're after big horsepower you would be better off with a slightly smaller snail.

The GT28BB or 200SX T28 (very similar turbos) will make 150 - 170rwkw on 15psi with basic mods (exhaust, intercooler, chip). This is plenty in an 800kg car for most people and the boost will come in alot earlier.

It might be worth having a chat on nissansilvia.com but be prepared to sort through alot of bullshit........

I have been through all this myself in the past 18 months. I settled on the GTi-R T28 and it is currently making 141rwkw at 10psi. In hindsight I wish I had bought the GT28BB. But your needs may be different. I love circuit racing and at the track I race at the less laggy turbo would be beneficial.
Regards Matty