Your average sr20det equipped drift car probably has between 200 and 300hp. There are however some exceptions. D1 drivers for example tend to have more horsepower than your average weeked drifter, so you'll probably find that sr20det-equipped D1 cars generally have more than 300hp.
KA vs. SR20...which gives the car a better weight ratio?
I don't have the exact weight of either engine (SR or KA), but from what I've heard on other forums, the SR is lighter and provides a better power to weight ratio.