LZMA2 isn't using all the cores, at least at the moment (see here), although that may change once I look into it. (Maybe it's like that for the reason you give.)
We probably won't add anything more granular than the "use multiple threads" option. If you want to specify exactly how many threads should be used, you can use 7-Zip itself for that. (But our thinking on that might change once I look into the LZMA2 threading stuff, if there are compelling reasons for wanting more than just on/off for multithreading.)