I'm using SDL to render them to a YUV overlay
XVideoTextureSyncToVBlank is not related to overlay video adaptor, but to video texture adaptor.
If you're lucky and you driver/gpu supports YUV overlays, try to use them!
with mplayer. you can speedup the rendering of opengl video (plus nicer scaling) using:
mplayer movie.xxx -vo gl:yuv=6:lscale=1:cscale=1
(it will still use some more cpu power compared to XV texture/blitter adaptor and, of course, YUV overlay).
You may also want to benchmark with different "yuv=[1..6]" to see what's better for your card.