Yes, I am the Performance Sales Manager for HTT. Since we don't sponsor any forums, I try to keep away from the 'selling' side of things and just try to chime in with answers to questions when I think I can help.
As for the power they'll support, it's really hard to say at this point as we just don't have that many out there yet and don't have dyno charts on them. The HTT S371 has done 800hp in an LB7, but that is pushing it way past what I'd say it's limits are, as the back pressure and EGT's would be pretty extreme with that kind of fuel. On paper, this new 369 actually outflows the 71mm wheel we've been using, so the potential is there, but with the turbine wheel no longer being clipped, I think that will be the limiting factor. The new SXE making more boost than the standard S300's do, and is spooling and responding better at low and mid RPM due to not being clipped, but it is a little more restrictive on the top end. It's going to be a much better turbo to drive however. But I would see no reason that 69mm could be used on a 700hp application and be fairly 'streetable' at that.
Have a customer in Florida running one on a 2014 6.7L Ford, crew cab short bed (8500lbs) and just went 12.8@107mph with only 1800uS. That motor is pretty similar to a Dmax and calculator says he's gotta be 700+ at the tires and he tows with it on a regular basis.
Both SDP Twins and Wherli Fab have a couple of the 66 and 69 SXE turbos they are testing in their kits and have given great 'driver' feedback, but have yet to get any dyno time with them. SDP will be running the 369 with a GT4718, so that should be interesting.