well, 1st post and you've got a lot to tell us.
2011, all the original parts or did you mix in other stuff?

Is it truly backfiring ( through the carb ) or is it afterfiring(through the muffler)? Did you happen to adjust the valves?( If you followed the Clymer's book ,and rotated the motor cw to TDC ,this is incorrect ,the motor should be turned ccw /or the same way the wheels turn when viewed from the left side of the bike,your valves may not be fully closing). How many wires did you hook to the new starter switch and in what positions and where did you run them to?  And more importantly ,why did you change from the stock switch? Seeing you still have the decomp ,timer ,headlight relay that turns off during starting,and you have to bypass the starter and maybe the kill switch ,all complicates installing the newer switch.There is also a wire from the starting circuit that runs to the ignitor to signal it to retard the timing at startup ,if it's not functioning that might cause backfiring,as the sparkplug will be firing to early. If I were you I'd go back to the stock switch and wiring ,as removing your hand from the bars to use this type switch reduces steering control and use of the front brake and throttle ,I see no advantages.

The switch change is purely aesthetic.  There's no more controls on the handlebars.  I have 3 toggle switches on the headlight housing (power, high beam, low beam) so the headlight is already off when it starts.  It did have 16 inch ape hangers so I had to shorten all the wiring to accommodate the lower bars.  The headlight brightness was improved greatly by shortening the wires.  I guess who ever did it the first time did not do so well.  Sure will be nice to see the road ahead of me at night again....   Ill post photos once it's done.  Getting the air box covers and gas tank powder coated.

awesome, I’ll be emailing you soon for your super fix when it’s time to make the switch