The overall average PSNR for ispc_texcomp was 48.57, mine was 50.4. Even with ispc_texcomp's massive mode and SIMD advantages it does worse on this metric. ispc_texcomp doesn't support optimizing for perceptual metrics, which puts it at a huge disadvantage on many texture types.
I re-encoded the textures with linear metrics. My encoder used 6 modes: 0, 1, 3, 4, 5, and 6 (including all component rotations and the index flag).
Note that ispc_texcomp in its best/slowest profile is pretty slow, and is much easier to compete against without SIMD code. It's just trying way too hard. It's faster in its lower quality "basic" profile, but it still doesn't support perceptual metrics so it'll continue to fight up a very steep hill.
For benchmarking, I ran each encoder in a single thread, and called ispc_texcomp with 64 blocks at a time.
Other findings: ispc_texcomp has a very weak mode 0 encoder, and it's weaker than it should be on grayscale textures. I'll blog examples soon.