No backpressure is not a problem its exhaust speed if the pipe diameter is too large for an n/a engine then the exhaust speed is lost and you loose the scavenging effect of having a high exhaust speed. Ideally you want the a high exhaust speed at all rpms but its impossible since you will either not have enough flow to create the "vacuum" effect that helps an n/a or too much (for the given pipe size) so it chokes on itself and you get large backpressure. There is a balance going on to get the most ideal for the engine size and rev range/airflow.
Now i am done ranting on about backpressure (its one of those things that gets to me) what you are loooking for headers with quality Y joints since (mandrel bent pipe headers are good to look for them). The stock ones kinda suck since they are made to help cancel noise. From there on your piping size should be about 2 1/4 inch or so a 3 inch will not do any good and too small it chokes up. I'm tired but the jist of what I'm trying to say is there.
