Hopefully the upcoming halos adress this adequately. I think that given enough time they would definitely bounce back and become a force to be reckoned with once more. The issue will most likely be if the unsc gives them enough time to do so. Left alone I'm sure they'd work back up into being quite powerful.
It'd be a pretty dark, twist if we started doing to them what they did to us. i think its more likely that the fighting shown in halo 4 will continue at an increasing pace. I can't honestly say who would win here. The UNSC has the technological and naval edge this time but unless we bombed them from orbit I think we'd have a hell of a time taking planets from the sangheilli. Even with the spartan 4s and new guns and stuff I think that thier sheer (shear?) numbers combined with the fact that they're fighting on home turf would give them the advantage in ground combat. And they tend to live in these fort/castle ish things. so they're pretty much ready for ground combat whenever.