I'm *extremely* skeptical you're going to notice any meaningful performance difference between VGA cards in an XT with 8-bit slots.
Here's a thread from a while back where someone was testing various VGA cards in a 486 class computer with 16 bit slots, and here's two takeaways:
1: The cards clearly cluster into two groups, and what separates those two groups is whether they provide 16 bit access to video memory or not.
2: Within each group the differences are extremely minor.
And, of course, with an XT point number 1 doesn't apply to you. Note a confusing point here: you might notice in the results in that thread the OAK OTI-037 was clustering with the 8-bit cards instead of the 16-bit cards, and thus looks slower than the TVGA9000. The reason for this is because OTI-037 cards with 16 bit bus connectors were living a lie. The only thing on the card that offered 16 bit access was the BIOS ROMs, the VGA chip and memory access was 8-bit. This means, at best, when installed in a 286 or better computer an OTI-037 might score a little better than a purely 8-bit card solely on benchmarks that exercise the BIOS ROM calls for printing/scrolling/whatever text. It will be completely irrelevant running almost any graphics program. The Trident *does* have 16 bit access to video RAM, which is why it clustered with the 16 bit cards. But since your XT only has an 8 bit bus I'm going to be *very* surprised if you see any difference between the two.
And regarding point two, sure, some cards are a *little* faster than others, but ultimately the limiting factor with ISA VGA cards is
16 bit ISA can't do anything faster than around 4MB/second at very, very best. Some VGA cards have less memory contention than others, so with a really fast machine that can actually max out the ISA bus, sure, you might be able to *measure* the difference, but... an XT isn't a very fast machine. Most XTs can't do a memory-to-memory copy any faster than around 500K or so per second. So even the very worst VGA card is unlikely to be a significant bottleneck.
FWIW, I have a somewhat more modern OAK OTI-077 in my Tandy 1000HX, and at the 7.16mhz "turbo" speed the video memory on that card benchmarks faster than the 256K of memory on the system's motherboard. (Which for complex-to-explain reasons is effectively strapped to run at 4.77mhz.) So unless the contention is much worse on the 037 I'd expect it to run as fast as an XT can push it.
