@Ronnie21093 >find the absolute FPS limit TH6 can still be cleared at
The answer is technically any so long as you feel lucky.

The real answer is you're going to have a frustrating to nearly impossible time on anything over 60fps. The games were designed with it in mind, logic and everything, so the more you add the more desync and random problems you get. Even just doubling the framerate would likely make most bullet patterns unrecognizable and some impossible to dodge (and you're not going to have any cheat tools like Seija has). Pretty sure things like replay logic and whatnot also work off this, so a difference in FPS could lead to impossible to verify replays.
I'll be honest FPS arguments have degraded so much over time it's genuinely mind-numbing to read any. FPS is a terrible metric to judge a game by unless it's actually creating a problem, such as impeding gameplay. If it hits a stable 60, that's more than good enough.