Just so you are aware, a stage 3 or 4 transmission doesn't really have much of a strength increase over stock (depending on what year you are starting with). The ONLY manuals that I would really trust long term against 700+hp is a full PPG or PAR straight cut gearset. He is right, the highest HP that any DSM/VR4/early EVO was ever asked/designed to handle was 240ish hp. If you compare them against the Evo4+ they are very narrow, and they use the 3rd gear as the drive for the center diff, putting much extra load on it. Now I'm not saying people haven't made xxx-whp and been fine behind Mitsubishi gears, but they are on borrowed time, and anyone who has been in the game will agree with that. I'm not sure on where TRE stands as far as continuing to build them, but a lot of it has to do with cost/expectation. Will a "stage 3 or 4" trans from any one of the builders shift well under high RPM/high load, absolutely. Is it going to never blow up just because it uses the best FACTORY OEM parts, of course not, but people assume because it's a "race" transmission and cost 4k it won't, but it will, catastrophically, I almost guarantee it. After 600-650whp depending on weight of the car, torque output, and amount of pulls/etc, 3rd and 4th gear become a gamble. No ways around it. Shot peening, micro polishing, cryo treating etc, will increase the window a tiny bit, but it's not a 2x increase or anything.
I'm actually glad he responded the way he did, not because you wanted to hear that, but because you needed to. If you want a dyno queen or cruiser, then it will probably hold together just fine. If you plan to track the car exclusively and make 7-8-900 HP, empty your wallet for an aftermarket gearset, or go Auto. Sorry. End rant.