Ah, spoke too soon. The reason the memory appeared slower was that it was at 17-17-17-whatever. The advertised timing for this memory is 15-15-15-35. When I entered those values in normally, it still threw errors in Memtest.
I noticed XMP info had TRRDS, TRRDL, and TFAW values that the BIOS was not using. I put in the correct settings (keeping XMP off), but still problems. I'm now trying TRFC 677 (535 default). XMP/SPD info gave TRFC1, TRFC2, and TRFC4 values instead of one TRFC. Someone else using G.Skill RAM saw that they used the higher TRFC number than what the BIOS was so I'm trying that.
If that doesn't work, next step is to try the presets. There's a 1.35v/3600MHz setting in one of them.
If that doesn't work, I'll try tinkering with VCCIO/VCCSA and Vdimm again. Right now, with Tweak Mode 2, VCCIO and VCCSA are at 1.208 and 1.27x (top value, I don't recall which, is 1.208, bottom is 1.27).