Denting/deforming a cap can reduce the voltage rating since the conducting plates have probably been squashed closer together. Dented bad enough could short the plates.
A cap with a rounded top means the electrolyte has expanded (from heat, overuse, etc) and is on its way out and should be replaced.
You are right, the more caps the merrier, but the more you add, the harder it is to stay neat while keeping the leads short enough. Plus, you get diminishing returns the more you add. For instance, if the ESC has two 330uF caps, that's 660uF total. Adding two more doubles the total capacitance to 1320uF - A pretty good gain. Adding two more increases it yet more to 1980uF, but obviously won't double the total value. However, the cap bundle is starting to get unwieldy and harder to connect close to the ESC.
No, you won't find these types at any RadioShack that I know of. As a matter of fact, if you ask them for low-ESR caps, they will simply give you the deer-in-the-headlights look and then try to sell you a cellphone.
Seriously, larger parts suppliers like Mouser, Newark, etc has them I'm sure. You can sometimes find some on eBay as well.