$300 - $450
Yes, slightly.
Assuming the basic, exhaust, FMIC, etc. 15-30rwkw.
To get the most and safely, yes.
The biggest advantage is you'll get more power on and RB25 Turbo @ 7psi than you will on a RB20 @ 10psi so you're not stressing it as much, when turbo exhaust wheels let go they don't always just end up as rattly bits in the cat, they can go back up your exhaust ports and in to the engine, happened to GTRGeoff & RowdyR32 just recently, not pleasant...