Changing your gear ratios will increase or decrease your top speed and acceleration, but it may leave you wanting more. You can almost gear the Mabuchi 540 any way you like without harm.

Of course a new motor is always exciting, but I dont know what type of speed controller you have, so I wonder if it can handle a "hotter" motor. Once you get to the point where you need a stronger ESC and you want a faster brushed motor, then you have reached the price point of a good brushless system that would give you increased speed plus runtime, and it can last many years!

If your ESC can handle lower turn motors I would recommend a good quality 19T motor. This will be significantly faster than the stock 540, but not so fast you lose lots of runtime.

I like the Nimh battery packs from Pro-Match. They are not overpriced, and they perform well.