In fact, rendering subtitles at the display resolution is one of the big selling points of the xy-subfilter + madvr renderer combination.

The only practical downside I have noticed is that accurate rendering of subs containing complex vector graphics or effects (ASS supports that) at > HD resolutions takes a lot of CPU time, sometimes more than a single core can handle in realtime.

There probably is a lot of potential for optimization, but those are hobby projects for their maintainers.