Andy.
I dont like to comment on stuff I have no experience with, I've not done an M30 yet, I have helped a little with Toby Unnas but that is very much his conversion, and very nice too ( still no like the TB's though.... )
from memory his car made 324HP on 6psi on a T4, boost came in at 3500 rpms iirc, awesome shove
Typically you'll want a turbo 2x the size of what you think you'll need to make good top end HP, but if its all used in one 5K to limiter burst, how good is that gonna be as a daily ??
The huge holsets work well as they are diesel turbos, so given much higher EGT of petrol, they will boost at the same point in the rev range as they did in the OE application,
Some very rough examples of why these work so well ;
Cummins 6BTA engine, 6 cyl 12V diesel, HX35W, boost at 1700, engine rev range 0 to 3400 rpms EGT of 300approx
so, in simple terms the turbo boosts @ 50% of the engines rev range
BMW 6 cyl M30B30 , 6 cyl petrol, same snail; as above on similar mainfolding setup, with EGT of 600C, the turbo should spool at 3.2K ish all the way to a 6600 rpm redline
^ see where we're going here ?
I assume your car is RHD, keep that in mind as a 3" downpipe is a hard squeeze and a very large turbo will require a large d/pipe bore to prevent it bacpressuring with some unwelcome effects
The turbo you posted @ top of the page looks to be OK in terms of dims, but its a much older design and possibly less suitable , but as you say its FOC , me, I'd bolt it up and find out
