I'm betting you are running lean. Try turning your air mix screws out and if that does not lessen your backfiring, up the pilots to 45. If you are doing away with the airbox, you might want to start with 145-150 mains and see how that goes.
The TBS needles have more of a taper than the stock needles. Changing to them would be like upping your main jet size slightly so it's an easy way to richen the circuit.
You can drill out the vacuum hole in the slide to 7/64" which will help make the throttle response crisper.
Please only do these mods one at a time. If you do them all at once and have a problem, you will have no idea which one caused the problem.
I'd work the pilot/low speed circuit first and then progress to the mid/high speed.
