Hey Casey!
I think that 4 12 cabs have a sound all their own. It has to do with cabenet volume and loading /damping on the amp as well as anything else, so even when you only mic one speaker .. it factors in. I suggest you have several different cab configurations around to use and decide from them which you want to use all the time or from time to time.

For crunch / metal / heavy guitar oriented stuff a sealed cab half stack is almost mandatory but for a lot of country, pop and especially blues an open back fender style amp perhaps miced from not only in front but from the back as well (be sure to check phase) is a great sound .. so it all depends on what your working on ...

you said something about upgrading speakers? What do you have now? I like vintage 30's myself. As far as it being overkill, it depends. If you are trying to get a monster sound, it doesn't matter if you are using a 4x12 or 8" speaker. It's all about mic placement and guitar sound. So I don't believe size matters much. I would say the only downside to 4x12, is that if you are recording live and don't have good isolation, you get a lot of overbleed, sometimes a lot of the low end. But whatever sounds good to you is what you should use. ALAWAYS trust your ears. I was at Indigo Ranch a couple of years back and Richard Kaplan showed me the amp that they used for the first limp bizkit record. It was an old kay amp with a 6" speaker. And that was the main guitar sound. If you listen to the record, it sound monsterous. Also, he showed me his secret weapon amp. The amp that most artists used in their recording (korn, slipknot...) It was a little old fender silverface champ. Just a few examples.

Thanks for the input guys.

I took the cabinet apart and looked at the speakers. They are Peavey Sheffield 290's. I can't find much info on these.

I'm guessing an upgrade would be worth it. I'll probably just buy a better cabinet.