Today i received the high speed gear set for my TT-02 and i've tried to install the 0.4 90T spur from tamiya and it works great.
Been using it with a 48T pinion on motormount position for the 0.6 23T pinion, the gears meshing up as it should be and even the noise of the car is a bit less then normal.
This combination should give your car a ratio of 4.88, it would be equal of a TT-01 using a 55T spur with a 29T pinion (but we all know that this combination is normally not possible)
So the TT-02 can be faster then a TT-01 car, this with a better handling car should give you a benefit at the track