Why 'com_maxfps 250x' uncaps Quake Live's frame rate

In Quake 3, yes. Here's a nice, short video on this: https://www.youtube.com/watch?v=he02vJvKaRs

To achieve 333 fps in Quake Live, you can unlock fps with `250x` and then use an external frame limiter, setting that to 333.

I haven't tested this, but I would guess that they addressed this bug in Quake Live.

Tho I still would expect there to be some physics-related issues and other oddities when running the game at 1000 FPS.