It is kind of funny that you say you don't want "added problems of a 3.6", but then talk about 3.7 twin plug CIS.
Displacement increases for 3.0 engines, in order of complexity/cost:
98mm P&C with stock crank (70.4mm stroke) 3.2 litres.
Stock 95mm P&C with Carrera crank (74.4mm stroke) 3.2 litres
Combination of 98mm P&C's and Carrera crank, 3.4 litres.
Larger than 98mm is possible, but you must machine the spigots on the case.
As to how much you can go with CIS, I don't think anyone really knows for sure (except maybe the guy from Andial). CIS is a mechanical system, with fuel delivery based on the position of the sensor plate. In other words, unless you can find out exactly how Mr. Wagner set up his fuel system and can replicate it, you are in uncharted territory. You may want to read
this thread for a discussion of high HP CIS.
Tom