When I run 4 instances of Handbrake the i7-7700K uses all 8 threads at 100% for hours and runs at 4.4 GHz (out of the 4.0 GHz stock speed and 4.5 GHz max speed). I don't OC, this is just stock on a H170 board.
I noticed when I had a 92 mm cooler it would drop down to 4.2 GHz every once a while. So I bought the 120 mm cooler and it stays at 4.4 GHz at around 70°C.
So yes, you get almost all of the turboboost. HWMonitor tells me it uses about 89W when doing so.
I don't have single-threaded application, so not sure if a single core really goes to 4.5 GHz. Even one instance of Handbrake lets it go to 4.4 GHz (except only shows 60% load on all 8 threads).