Mike, don't underestimate flow capability of a T3 turbine. There are several sizes of T3 turbines to choose from, Stage I (one) is the smallest and stage V (5) which is the largest. The stage 5 is nominally only 0.125" smaller in diameter than the TO4 middle size "P" turbine, yet about equally larger than the small TO4 "O" turbine. Also note that the Stage 5 has an outlet dia which is about 0.100" larger in diameter than a K27 turbine. The T3 Stage 5 has far less mass and will spool faster as a result. It's really worth seeing the two side by side to understand how little mass the T3 turbine has compared to TO4 turbines. I will say one thing though, you have a really BIG displacement engine, so you could be headed for TO4 gear, but the one size up from P is the Q turbine and I believe it's intended for V8 size displacement with a single turbo. I recall your recent posts where you mention street drivng response was your main concern so perhaps the stage 5 will suffice. If not, sell it and buy an alternate. This is a drop in the bucket compared to what you have invested in your engine. Call it R&D. I used to spend $350 on a housing, now the whole turbo only costs $750! Turbonetics that is.
On a different but similar topic I would like somone WITH turbo manufacturing history to chime in and clarify. I started building custom turbo datsuns and toyotas starting in 1982 or 83 through to 1990. Over the course of time I bought a number of turbos, rajay, Garret, Schwitzer, but the most versatile brand was Garret which had the widest selection of domestic components and supporing tech info. The Garret name eventually disappeared and was replaced by, non other than,... wait...wait.... hold on....
TURBONETICS! Same products.
Now, a few years go by, and guess who's back in business! GARRET!
So, what the heck happened?
Note that Turbonetics was selling split vane BB turbos back in about 87, and in the prior version of Garret there was even a bunch of BB with variable vane but these seem to have died due to reliability of the vane mechanism in the harsh exhaust environment. It really comes down to a match of components to achieve the result you want, and turbonetics gear seems economic, and has some history. I don't think the products are flacky, but the marketing may be.