Just going from a 2.5 to a 3.3 traxxas motor with dual chamber traxxas resonator pipe will already be a big step up. The 3.3 is really that much better than the 2.5. It's got heaps more bottom end and maybe a bit less top end, but you can make up top end with gearing. And also, the 3.3 is easier to tune, and runs cooler than the 2.5.

You can also get the 3.3 from ebay "chop shops" at a decent price.
That would definitely be the route I'd take.