I replaced the timing belt and pulley tensioner about 300 miles ago and am not quite sure if I did everything right.

Problems I've been having:
1) After belt change the rpm went up to 900rpm (manual transmission)
2) Poor performance, feels a bit sluggish (I have the b320ft engine)
3) Poor gas mileage, around 20miles/gallon mixed driving (I usually get around 23-25 mpg mixed driving)
4) Sounds like backfire while downshifting (but may not be a new problem)

Why I think I did the timing belt change correctly:
1)When I put on the belt I alligned all the marks (and after running engine and positioning the engine to tdc, all marks remain correct)
2)Timing light shows correct timing at 12 degrees before top dead center (however it almost looks like 13 or 12.5 degreess btdc, hard to tell exactly)

Does this sound like I messed up somewhere or am I just convincing myself I screwed up? Has anyone had similar problems? Any information is very much appreciated. Also, thanks to everyone that has helped me in the past, probably the reason my car still runs.

Thanks,
Benton