Personally, I am not at all against raising the bar for baseline x86_64. Of course, it'd be ideal to have some sort of derived x86_64_avx arch, but if we find out it'd require too much of an investment into infra/releng, I'd be +1 for just changing the base x86_64. Sure, it'd make sense to actually see some numbers from Fedora compiled with SSE4/AVX/AVX2 and not just guess from Clear Linux results.
I see AVX2 is just too high baseline (although, all my PCs and laptops support that for at least 2 years), but raising the baseline to something like AVX or SSE4 might make sense. I don't know why people with *not ancient* computers should have degraded performance just because we want to support everything from K8 from 2003. But as I said, it'd be nice to see some benchmarks to base the decision on and have optimized x86_64 as secondary arch, if possible.
* the performance increase to be had is marginal, given that we are mostly
talking about code written in C or C++ without even compiler vectorization
(-ftree-vectorize) turned on,