I have tried the idler mod once on a hi-grip indoor track, but I think it is best suited for outdoor low-grip. Didn't like it at all.
Gives the car alot of forward rear grip when accelerating but vice versa when under brakeing. So stay with the 3-gear tranny.
Have you thought of a centralised gear-box brace not including the motor plate? Otherwise you will get different chassis-flex left to right.