For the 86 Trueno,
A, B and C tunes get the AE101 racing engine at 120k. This engine gives great accel in the upper gears.
D Tune gets turbo at 120k. This gives good accel in the lower gears, giving better start-up but possibly lower top-end.
N Tune gets the normal engine.
For N-Tune, yes you get internal upgrades exactly the same way as you would the other tunes.
Turbo cars usually have a faster start-up, so you can say they have an effect.
The RX8 is a 6 gear car.
I hope that answers all your queries..
