First, welcome.

It's not strictly necessary to change the exhaust or jejet, though you can gain a few HP easily that way, which may well be appreciated on the interstate.  If you are happy with the current power, I would say you really only need to adjust the pilot circuit (idle mixture) and midrange circuit (white spacer) to rid yourself of the worst lean conditions.  That will allow your engine to run cooler and decrease the backfiring.  These can be done for little or no money, though I'd suggest replacing the carb's screws with stainless allen screws which would cost a few extra dollars.  If you want more power, then open up the airflow by upgrading the exhaust and air filter and increase your jet size to compensate.  Lots of instructions are all over this site.  The cost will depend largely on what air filter and exhaust you choose and what kind of deal you find.  The jets are about $3 - $4 each, so they're minimal.

Lastly, you may want to address the cylinder head seal/plug as it will be an issue with your model at some point if it hasn't been fixed yet.  However, it's usually more of an annoyance than a real problem.  It's fully covered in the Tech Ref forum.

I have a 2000 too, and other than a K & N filter, and jardine exhaust, I haven't had it jetted yet.  Will check into that later in the summer, if it's necessary.  My bike does backfire, but I can prevent it most of the time if I don't throttle back quickly, etc.  

Anyway, I've been on the highways here, and had it running at 110 - 115 km/h with no problems. (65 - 70 mph approx.)  Gas mileage seems to be 4 litres of gas = 100 kilometres of travel.  (@ 60 mpg)