I installed catback and my car was backfiring a little too. recently I added highflow cat and it got real loud but I don't mind. so far I run 87 octane gas. on my next fill up I'll try 94 and see if there are any changes. I guess the backfiring is normal then
The higher octane will probably just provide more backfiring. Besides, you don't need higher octane gas unless you're runinng higher compression, or an ecu that uses advanced timing (i.e. the MP3 ecu).