In my opinion the AA batteries are a limiting factor even with the stock motor. AA's are usually better in lower amp draw setups and a rc vehicle is a comparatively high amp draw setup. I'd try making the switch to something like the link below first. I believe they're called sub A cells, either way they should be able to take higher amp draws while maintaining a higher voltage under load. This means more torque and rpm potential.
Choosing a brushless motor isn't always easy but you can usually get pretty close to ideal. Then you fine tune the gearing to make everything perfectly happy. If you pick a 5000kv motor you'll have to run a smaller pinion than say a 4000kv motor. Obviously the 5000kv is going to be spinning faster but the smaller pinion will keep top speed the same as a 4000kv with a larger pinion. If you're going to be running 9.6v you'll want to stay on the lower side with kv. Not really that familiar with the mini inferno so can't really make any kv recommendations. I wouldn't run brushless on AA's though as amp draw is higher from a stop than it is with brushed.