800kHz for WS2812. I've seen references that say they might work with the 400kHz timing, but haven't tried it myself. Strips that use WS2811 controllers separate from the LEDs work at 400kHz. And of course, the WS2801s are SPI and you control the timing via clock signal.
And yeah, I think that if the signal degrades too much for that distance, a buffer could do the trick. It shouldn't be hard to find one to handle the speed, but you'll want to make sure it can also handle the current requirements. From what I understand, the WS2811/2812 modules themselves act as buffers, and re-condition the data signal on output (if I'm wrong, somebody correct me). So as long as the signal coming in is good enough, stretching things out shouldn't pose too many problems.
