WEVO and Vellios are two different outfits. I'm not sure if you're saying you had both setups at different times, or different parts from each at once.
Vellios made replica 916 kits, which I think turned the 915 gearbox into a side-shifter like the 73+ 914 gearbox. Quality evidently varied a lot from piece to piece.
WEVO not only does reinforcements and modifications for the inside of the 915 box, but they also do a nosecone that mounts a 915 in a 914
and has a gear selector sticking out of it for you to hook to a new fabricated linkage. I have not used this setup, but I am pretty confident that it will work well--because the WEVO guys are really very good.
I have driven a 914/3.6 with a 915 box and a home-brew shift linkage. It worked reasonably well--at least as good as the "well-loved" stock 914 side-shifter that I was also driving that day. (The latter was my own car.) It was not a Miata shifter, but I don't think you'll get that in anything but a Miata.
All of the setups can be made to work with some level of effort. They will all work reasonably well if you put the work in on them, at least for a while. For applications with lots of torque, though, the 930 gearbox does seem to be the strongest reasonable option.
The Boxster trans seems like a cool setup, but again the installation is non-trivial. I'm also not convinced about how much torque it can live up to in the long run.
The original goal of a 200 MPH 914 is an interesting thought experiment, but to me is a total non-starter. Especially a streetable 914. You need a lot of down-force to keep the car from flying at those speeds, and that means drag. And even a little drag becomes a really huge amount at 200 MPH, so the power requirements would be pretty astonishing.
--DD