If it's mild backfiring that might be ok. Your new exhaust has little back pressure and scavenges the cylinder better. This can pull raw air/fuel mixture in the exhaust on de-cel under load. This will cause mild backfiring. If it's loud then that's not good.
You might want to check for vacuum leaks. I had a tear in my brake booster diaphram. The thing sounded like a gunshot on de-cel.
Do you have tail pipes? Any droning?