best upgrade?
 

I have a question for you guys. I recently purchase the ST-1 in RTR form for bashing in the backyard and with some friends on a home made track. I've put some 1/2 offset wheels on it and purchased a 14t cluthcbell that I havent installed yet and thats all I've done to it. My question is, what would be the best upgrade I could do right now that would have the biggest impact on performance or handling or just best all around upgrade I could do? I have about $100 so I've been considering picking up an Airtronics MX-3 tx/rx from tower or maybe a used one from ebay, I've been looking at the Thunder Tiger 3 stage tuned pipe (long) for bottom end and I've even considered a new sterring servo even though I can't "perceive" any performance issues with the stock servo. What do you guys think? If I try hard enough I could probably pick up a used radio from ebay and get the tuned pipe from Amain for $100 total but it might take a while. Let me know what ya think, thanks.