Like Tex said, better tires. ($100)
Then snyth radio if you're going racing so you're not "waiting for Blue". ($150 used)
Then P2 rockers, push rods, & springs for better handling. ($40)
Then stronger single steering servo ($120) and linkages ($40)
Add a wing in there somewhere along the way. ($40)
New body ($35 - painted add another $30-60)
Misc spare parts ($ ????)

Should be about $1000 by the time you're done with your $425 Revo. Dontcha just LOVE this hobby!!!!!!!!!!!!!!!