Don't forget to check out Wolf - they beat every other brand (including Akrapovic, Devil , Blue Flame etc) in a back to back test done by Motorcycle News earlier this year - voted best quality and sound etc even though they are half teh price of Akrapovic
Click the sponsor link on the left and have a look!
There are better pipes out there than the 2 Bros but not "significantly" better - unless you really hate it , it's not going to be worth getting rid of a freebie to spend several hundred on a new can... you might get a bit better quality, a bit deeper noise and maybe a bhp or two but you'll have very little to gain and the money would be better spent on nice sticky rubber!!
for my '03 i would probably buy the wolf underseat model. i like the twin cans under the seat, but i dont have the map for it so. with all the maps out there so far i would more likely go with the akra.