Here is a page from turbonetics on turbine sizing. On their web site is a great deal more technical information but sufficed to say I don't see anyone considerating a T3 turbine for 3.0 liter applications and up. Hence a T3/T4 hybrid is out.
Regarding a twin on a 911SC, I believe there is a thread on that but I have no real expertise in twins other than having an extra KO3 from my 1.8T that I'll give away to see a twin built.
I would say it needs to be sized for a 1.5-1.8 liter and a standard exducer KO3 would be close.
Mechanical CIS or EFI should not make a difference in turbo selection.