After testing the Ryzen 7 1700X with SMT enabled and disabled, we can say that disabling it can improve performance in some programs. From the games we testes, Hitman and Deus Ex: Mankind Divided had a small raise of the frame rate when we configured the CPU to stay with eight threads, as well as with Media Espresso and DivX Converter.
However, in programs that use all the available threads (like Cinebench R15), there was a significant performance loss when SMT was disabled.
This means the decision of disabling SMT or not will depend on your usage: if you only want to play games, it can be a good idea to disable the feature, since none of the games we testes took advantage of it. On the other hand, if you use your computer to video or image renderization, or any application that actually uses 16 threads, it is better to keep it enabled.