If you are even able to order and afford the 3990X you need to spend close to 4000 bucks. With 64 cores and the default temperatures we have seen, we made the call to at least try a bit of tweaking, but not with added voltages in order to keep that temperature in line. Now, obviously it is a little silly to overclock 64 cores with an all-core tweak, but you can and let nobody tell you that Threadrippers cannot be tweaked. Normally your all-core clock frequency will drop towards 2900 MHz if there is load utilization on all threads. So even tweaking all cores to 3200 MHz, would help in such a scenario. At default voltages, we reached a stable clock speed of 3.5 and even 3.6 GHz (on all 64 cores). We are, however, able to boot into Windows and run some tests (not long term stable though!) at 3800 MHz.
Let me reiterate that overclocking a processor like this we find a no-go. We applied a simple 240mm LCS kit from Enermax. We previously scored 24.6K points in CB20 MT and now reach almost 30K points. Substantially we did score an extra ~20% in performance.
What will scare the living bejeezus out of you however is the 'Ryze' of power consumption with all 64 cores @ 3800 MHz and full load. That's double what it was initially. So the question is not 'can you overclock Threadripper processors', but rather, 'do you really want to overclock?'.