Hugo,
we want to slow the speed of the transmission and mainshaft down. if you use 18 the FDR is 2.80. if you can get to 20 the FDR is 2.40. what we are doing is slowing the mainshaft down so we have to gear the engine lower. The transmission is the heaviest rotating part of the car and it takes the most power to spin it up and down between gears which eats power. (inertia is the enemy) If we don't spin it up as high between gears its all free power that is not wasted spinning up excess rotating inertia. the difference between 2.80 and 2.40 is several thousand less rpm on the mainshaft between each acceleration and deceleration of the transmission.
However using 20's on the mainshaft with 18 as a first gear pinion will not allow you to gear low enough. (on stop and go USA type tracks, this is less of an issue on large sweeper tracks like europe has.) for 20's you really need to go 16 or 17 on first gear which you cant do on the G4s clutch bell as it is. However this can be done with a modified kyosho UFO bell and modified kyosho pinions.