We've seeing already many enormous issues. For example MSAA with ANGLE has CPU path, and is unusable at all, as performance drops insanely. ETC2 was another case where CPU path was pushed a lot.
Update: The WG is planning on only exposing the extension where there
is 'native' support. (Not D3D, maybe not Desktop NV?) We feel this
best matches what devs expect when they see support for a compressed
texture extension. Compressed image formats are a better delivery
mechanism than compressed texture formats, if they're going to be