Loss of hyper-threading is only "20%+" because you put a plus there.. Hyper-threading in synthetic workloads is usually in the order of 40% loss in performance. Depending on how well used the hyper-threads are it can actually be higher.
It's unlikely to be lower than 40% unless you're bottlenecked before the CPU (memory speed or heavy CPU cache invalidation)
Yeah it depends on your workload. If you are playing video games with well optimized C++ code that also takes advantage of SIMD etc then it's not going to matter a lot. However if you're running a python webserver that causes a cache miss every other line of code then this could actually result in a 40% loss of performance.
It's unlikely to be lower than 40% unless you're bottlenecked before the CPU (memory speed or heavy CPU cache invalidation)