I run a Yoshimura R3 4-2-1 on my 01......bought it on ebay for 520$ runs louder than my stock pipes...., fits like a glove, no problem with oil filter, weights about 1/2 as much as my stock set up did and can still use my center stand.
Go with a full system, it's well worth it...how ever if you go the slip on route, get the larger oval pipe. I saw a bird this summer with small oval slip on's and I thought it looked like crap.