A properly multi-threaded application is going to benefit from hyper-threading simply because it can utilize the extra logical cores just like it can the extra physical cores.
The real world benefit seems to be around 10%. Hyper-threading will do absolutely nothing for a single threaded application, and many report worse performance with it on, than off. As dx12 is supposed to be fully parallel, it should do the normal 10% performance boost that seems to be prevalent in benchmarking these days. It can be a substantial performance boost if you have too much going on for your available resources, but odds are you wont notice much in most games as few things actually task the CPU these days.
