I have one with a .7 cover and a RB25 Turbo read ground out to fit.
Comes on full boost at 3200rpm in second.
I'd go for the .6 compressor cover - Your max efficiency will be on lower boost. With the .7 compressor cover, max efficiency is at about 22psi - which is too much for a standard internal RB25.
So yeah, .6 compressor cover, and about 16psi and you should see over 250rwkw easily. If you want a bit of lag and heaps of top end, go for the .86 rear.
just make sure they don't sell you one with a to4s compressor wheel.