Overclocking is doable with SXOS. The retroarch menu for that works well. No need for external sysmodules.
Most n64 games run fullspeed, but some need a small overclock. I cant notice any differences in performance between sxos and atmos.
You might prefer PAL ROMs as they target 50fps. If you load NTSC roms, they need 60fps to run fullspeed which is harder to reach.