What's goin on man.
I have a 96 EX500, your mileage sounds low. I get almost 60MPG. You can calculate it by topping off, resetting the odometer, run about 100'ish miles (try to get to the bottom of the main tank) and then top it back off. Divide the gallons into the miles driven (on the odometer).
Generally, people Jet the EX's if they make exhaust changes. I don't know the symptoms of lean or rich so I don't know what backfiring is an indication of...
Depending on how I'm driving (casual or hard on it), I shift either at 4k or 10k. The power on these bikes doesn't even start until 7k but when it kicks in, it's pretty healthy. I generally cruise around at 4k, but that requires me to downshift 2 gears if I need to pass or accelerate quickly so I really just have developed a feel for where I need to be and when. I never cruise under 3k RPMS though because, from what I understand, it's WELL below the power curve and it's just hard on the valve train to try to make power that low.
Hope this helps. If you want some EX500 specific information, there's a forum for you.
http://www.ex500riders.com/forums/index.php