My understanding is that the HSTX can do twice the bit clock as the frequency of the CPU, as opposed to 1x for RP2040. So 800x480 (60Hz) is possible with no overclocking (295MHz bit clock) and Luke said [1] he got 1280x720 50Hz with overclocking (530MHz using CVT-R according to the video timings calculator [2]).
[1]: https://mastodon.online/@wren6991@types.pl/11292855122145752...