Well with the automatic version the bottom say 20% acts as (ATF) gearbox oil cooler so you loose that in cooling capacity, and the E21's radiator is already not overdimensioned. NRF, Diederichs and Van Wezel all supply manual versions under 200 euros as an alternative. But you can of course first try what you already bought...
Understeer is when you hit the wall with the front of the car. Oversteer is when you hit the wall with the rear of the car. Horsepower is how fast you hit the wall. Torque is how far you take the wall with you.