Windows and Linux have a much larger variety in hardware and drivers and much lower prevalence of high-dpi monitors. So we felt that for now it would be better to leave it disabled by default. This may change in the future.

Can you at least turn it on this time? I stopped using ST because scrolling was so incredibly choppy on linux. It always lagged a second behind and felt so unresponsive.

Edit: just read the patch notes, the ans is yes. Awesome!

Is there a DPI threshold that would benefit most from GPU support?

As always it depends on the hardware. I've personally found that things start to get slow at >1440p.

Is there a typing latency penalty to GPU rendering?

Latency in fact would be reduced due to faster rendering. Remember that it needs to go to the GPU eventually anyway in order to output to a display.