Back when I got an i7-3770K (4C/8T), I did a very basic benchmark using POV-Ray.

Going from 1 thread to 2 threads doubled the speed as expected. Going from 2 to 4 doubled it again. Going from 4 to 8 was only ~15% faster.

I imagine you could probably create a contrived benchmark that actually gives you nearly double the performance from SMT, but I don't know what it would look like. Maybe some benchmark that is written to deliberately constantly miss cache?

Side note, I should run that POV-Ray test again. It's been years since I've even use POV-Ray.