Advantages of extra capacitors?
 

What are the main advantages of extra external capacitors on an ESC? Would they be a big improvement on my MM ESC?

The increase power and decrese stress on the batteries. They are like mini batteries that store up charge when you are on the brakes or sitting, then when you nail the throttle they discharge giving you more punch.

What if I added, say, another 2-4 to my MM? Would that be overkill, or bad for it in any way?

If you add too many/much you'll get a big spark when you plug in your battery. Go the the top speed estimator and there is a "No spark calculator" on the left side. That should help..

I have heard 1 of mikes 2200uf low-esr caps is enough for a mm, but don't quote me on that...

I was thinking of using one of his 1200uf caps.

Whoops... That's what I meant. It should be sufficient.

Capacitors of large enough capacitance can do that. But with RC, we typically don't use big enough capacitors to really do anything (noticeable) in terms of punch/power. That's the job of the battery(s).

Adding capacitors to an ESC generally will lower voltage ripple provided low-ESR capacitors are used, and they should be of lesser capacitance so that they are able to charge/discharge fast enough to be able to keep up with the switching rate of the ESC. Capacitors can reduce stress on the ESC when the batteries aren't good enough, and can also be used in setups in which longer battery wires are required.