Yes that is safe. I used to have a 5820K. Should be fine up to about 1.35v
That said, 4.5ghz is pretty good for that CPU. The highest I ever got mine was 4.6ghz.
Your CPU is a great CPU. 6 core/12 thread with quad channel ram support. No reason to upgrade for gaming right now. Are you running your ram in quad channel?
I wouldn't bother messing with the cache. If you have it stable at 4.5ghz that is already plenty good enough. I don't think adjusting the cache will really do anything.
i7 7700kK @4.2ghz
16GB DDR4 3200mhz
GeForce 1080 Ti