Either pull out your precat o2 sensor or drop 1 side of your exhaust and try to see if you rpm level gets to more than 3000. If so you have a plugged cat. My cat did this when it went and when I changed it out I got full rpm back.
Backfiring usually means vacuum leak. Biggest offender is intake plenum gasket. It should look oily inside but since you've been pouring gas in it, that's maybe not so obvious. Stop doing that, by the way. It's a dry intake. You're not supposed to pour gas innit!
I agree with Gen1, to an extent. Mine had an issue with backfiring in the intake that's why I originally asked where it was backfiring from. I wound up changing the injectors because mine wasn't getting enough fuel (and therefore too much air) which is the opposite of a vacuum leak...I think. It ran decent when cold but as it started to warm up it went downhill fast.
Knowing where it's backfiring from is a further step in the right direction.