The CG of the TC5 might be higher than other cars but it is basically the same. Moving the layshaft and top deck down 2-3mm is not going to make a noticable difference to your handling.
A bigger difference would be cetralizing the motor. These parts are available from aftermarket companies so give them a try. As a bonus the guys who make the bulkheads lower the layshaft as well.
I have noticed a couple of things at my last two races that will make more of a difference than any small modification like lowering the CG:
1. Preparation (or lack of it) - I am surprised how many race cars are in pretty bad shape with strange toe-in/toe-out, uneven camber, poor diffs, bad shocks, dirty, etc. Any R/C racing car has to be prepared correctly or it will not work as well as it could.
2. Setup - Along with poor prep comes unusual setups. The setups that are posted by the team on AE's website are proven good fast setups. Sure they may not work for some people and might need fine tuning. But they are a great place to start.