24use 91 or bust
91 doesn't exit. There's only 90 and 95 for PCs.
edindx8 is simply better
DX9 is the latest graphics API, with more efficient resource handling. The only reason why DX8 gets more frames in some cases is because it can't support rendering some things, so it uses less advanced graphics. If DX9 were able to fallback to those simplified graphics, then it would have greater performance. But simple FPS comparisons are not what you should base things on anyway. Due to DX9's better technology, you get less stutters, smoother frame rates, and faster loading and window resource grabs.
DX8 is also a huge technological burden to maintain for the TF2 team. If we get usage numbers down for it, that would be a possible graphics pipeline they could drop support for, and thus improve efficiency by removing all those outdated pathways and not worrying about compatibility.