Well, the TT and the GMS are not usually seen in use in the competitive SSC contests.

I've heard the TT just doesn't produce the power with the 8x3 prop. With the SSC weights, it's important that your engine perform near the RPM limit. If you're only getting in the 15k range, you're going to suffer in the air compared to the engines that can turn 17+. Some SSC designs won't fly at all well with an engine running that poorly.

I've not seen or heard anything of the GMS .15.

Magnum used to have a bushed engine, the GP series. I'm not sure if they are still making them or not, but if they are, ignore them, it's the Magnum .15XL that is being used.

The OS LA is the easiest of the engines to set up and get running right.

The Magnum has a carb that needs a little TLC, but once you get the hang of it, it works fine. The guys who use bladder tanks find they need to put in an OS needle valve as an RNV add-on, since the Mag needle doesn't have a fine enough adjustment. Using a normal tank, I find I can go from too rich to too lean in 2-3 clicks, it's touchy.

The Megatech I'm not sure of, the one's I've seen run well, but I haven't heard enough or seen enough of them running to really feel that know what they are like overall. No shortage of power with them, though, that's for sure.