Hi,
I had the same difficulties. I was really good with rockets in IL-2, but when it came to SF2, I was completely crap. I think part of the reason for rockets dropping so much in SF2, is caused by the fact that you are traveling at a much higher speed than a prop driven aircraft, thus the rockets will decelerate faster (dropping as an effect) in relation to the speed of the aircraft. Its the same with a lot of the guns. I found that if you aim the gunsight a ways above the target, that usually your rockets will hit closer to the target. Also remember that rockets are a bit hit and miss; they work good fired in bursts, but not so good as precision weapons.
Hope that helps!