Actually I have a Dell Precision 7510 with the i7-6920HQ (2.9Ghz) CPU and it does stay at a sustained 3.4Ghz for extended periods of time at least when I'm docked, and even in my lap sometimes. It can be done I just dont think it happens that often because of thermal limits are hit too often. Thankfully my chip is cool and get a nice spead perk out of it.
I concur: my old 3630QM [email protected] (from 2.4GHz) on 4C/8T for hours under P95 (it never hits 90c...)
6920HQ has max boost of 3.8GHz, 3630QM has max boost of 3.4GHz so they work at higher boost but not at max. The same is with other processors. They go to higher state but won't work at max except short period of time.
Max boost also depends on temps and some other factors so many things may affect actual frequency.