iim not very familiar with this kind of stuff so i was just wondering why 18' would affect performance?
did you ever have a moutian bike? Larger wheels is like using the bigger front chain ring, its a bigger gear, and hence harder to push. Your top speed will be higher, but its harder to accelerate. You can get a 18 inch rim, and still maintain the factory wheel size, but metal is hevier the rubber, and the added unsprung weight will slow you down.