This is interesting. The mitigations have had a major impact. Especially with the recent loss of reptoline[1] on older AMD/Intel. Maybe enough that there isn't an advantage anymore to enable SMT on some processor families?
At an old job, we disabled SMT on our hypervisor hosts in an effort to mitigate some of the security issues before patches were available. We had to roll back. Without SMT, the hosts struggled to keep up with load. If I had to quantify it, I'd estimate that we lost, at least, 30% performance by disabling SMT (probably more). Those servers were replaced before I left, but I'd be interested to compare disabled SMT to enabled SMT with all the current mitigations in place.
Perhaps we're getting to a point that both disabled is faster than both enabled (and suffering from the mitigations penalties)