I have a bit larger bike I have tried using a flat cog style belt to compare to a sealed roller chain mind you this is final drive not in the transmission. This set up lost a average of 1.25 to 3 mpg on a 83 550 Yamaha Maxim. I tried thinking the weight reduction would help however it didn't. I put back on my chain after trying several adjustment. I even went as far taking drive pulley off and machining as much mass off as i thought remotely safe to do.
http://www.cyclingnews.com/features/...ve-whos-faster found this link when i was trying to figure why.