These are my recommended upgrades, on level of importance:

1) are they on ball bearings already? if not, I'd suggest this upgrade.

2) then the suspensions, it is on adjustable oil-filled suspensions? if not, replace those friction suspensions. even plastic adjustable oil-filled suspensions are way much better than the friction sus.

3) a set of springs and a set of oil (different thickness) for the suspension you can start with those set that have low-medium-hard

4) sets of tires with different foam inserts ... so that you can play around with the right one depending on the surface of your play-ground... use a good CA to glue those tires (highly recommend the Tamiya CA)

5) servo saver that will really protect your servo

6) a set of different pinions or gear upgrades to make you play around with your final-drive-ratio (FDR). different track configuration will require different FDR to make the best lap times.

7) metal gear servo with a fast response (that your TX can also handle - it is useless to have a fast servo if your TX sends slow signal response)

8) better ESC that will maximize the black can. Some low FDR settings can burn a black can or overdischarge your lipo. so upgrading them would be good.

9) commutator drops to clean your comms before any race

10) tire compounds - some surfaces gives better tire grip if the tires have been wiped with these tire compounds

11) if you will not retire from using brushed motors, a good motor tuning tools is also a good investment. this will allow you to break-in your motor properly, and can be a good tool to dyno your motors on hand and be able to pick which ones is in the best condition.

12) tire camber and alignment tools... very helpfull in race conditions and further augment your suspension set-up to maximize the stability of your car on those straightways and corners.

13) thread lock (blue) ... so woudn't have to worry about loosened bolts during your race. I had several experience that my car was unstable or got into a crash because of the bolts have loosened.

14) brushless motor and ESC set ... if you will graduate from the brushed motors and if the race you will be joining will allow brushless motors.

15) bling-blings - like those blue TRF upgrades

16) personal transponder - if you got hooked into racing big leagues.

17) tire warmers - to warm-up those tires before the race. just like F1s... hehehe

18) a new/branded newer model TX/RX set ... if you have reached the level of addiction to RC hobby... you will understand why I am recommending this upgrade

19) a different RC chassis set and bodies... because by this time you won't be content with one type of RC to play around with.