changing the pulleys in a naturally aspirated car will do bugger all.
an engine needs a certain ammount of back pressure to crate torque, too much back pressure and the engine will be struggling to put out much power at all and if there isn't enough its the same story.
one way to get near the optimal backpressure is to go for a full exhaust system - extractors and everything. or you can just make small improvements such as a cat back exhaust which is whati have. mine is a 2 1/4 inch cat back system, i think the engine feels a bit healthier for it, seems to be doing less work for hte same results.
back pressure isn't really something to worry about unless you a building a very high performing engine though.