I was similar to yourself firestonex, in that I wasn't sure about custom firmwares at first. However, in the end my curiosity got the better of me and I tried out a few.
If you are purely wanting to use the 7800 for emulators then you may find you don't need to change from stock. One of the reasons I upgraded was that I ran out of room for apps. This really does happen quite quickly if you start adding android games. However, emulators are relatively small apps and let you store the roms on your SD card, so you won't hit this limit with emulators alone.
I have never noticed any great speed increase using custom firmwares, although admittedly I am reluctant to try the upper limit clock levels. Stability counts for a lot!
On stock expect:
PERFECT
Atari 2600, Zx Spectrum, C64, NES, Master System, Gamegear, Lynx, Gameboy, GB Color, GB advance, Genesis, SNES, Others of similar era
Some Very Slight Sound / Graphical Issues (but totally playable)
Nintendo 64
Dreamcast
NeoGeo
Nintendo DS
PSOne
MAME (totally dependant upon the games)
Requires Frameskip
Sony PSP
Forget it, regardless of firmware!
PS2
Gamecube
You won't regret your purchase based on emulator play. Although I have a powerful PC, XBox1, 3DS etc, I spend an inordinate amount of time with this JXD