i think brief flashes would be a best of both worlds approach. - you could get away from the "oh his face melted by lightning" excuse, but also no need to worry about his difference of appearance.

Would the face stay like that up until Windu's death (as in, would it be the permanent change?)

that could also depend on just how dark [in terms of light levels] the entire fight winds up being.

Also it could harken back to the days of ESB when using the force required effort and focus - ie: Yoda lifting the xwing out of the swamp.

cutting down 4 PRIME Jedi can't be an easy task for an old man.