A lot of people like running the large FFT test. It stresses the CPU a little more and the memory a little less. Personally, I like the "blend" test as it stresses the ram better than large FFT does and the stability of the computer depends not only on CPU settings alone. It think the blend test is more balanced.
I would only lower the CPU voltage if you can do so and still be stable at you current CPU frequency. If you can get higher than 3.8 and do so without exceeding 1.5 vcore in Windows at idle or 1.55 under load, then you might consider that as well. I get uncomfortable when the CPU voltage exceeds 1.5 at idle in Windows or 1.55 under load, even when core temps are still good. I think exceeding these levels of vcore puts components at unnecessary risk of premature failure while only gaining a pittance more of performance.