If I were you, i'd leave the gear diffs in. Those Tamiya gear diffs are sealed so well they will never need a rebuild! I compared them with ball diffs and didn't feel a major performance difference. I doubt the speed increase with the lightweight outdrives will be significant.
Also, the Tamiya plastic shocks are one of the smoothest plastic shocks available. To save money, i'd leave them in as well. Besides, I prefer working with preload spacers than collars.
A better option is the updated carbon chassis. The stock plastic chassis is HEAVY and makes the front belt tight. The *updated* carbon chassis fixes these problems. A carbon upper deck will make the car look cleaner as well.
The one-ways will help in proper conditions.