2007 VTX 1300c - Unable to accelerate without backfire

Bought the bike in 2009 with 2400 miles, now have about 19k and kept up with routine maintenance every 4k at reputable dealer (Team Powersports in Garner, NC).

*About three years ago drilled out pipes (loved the results!), added K&N filter for extra air flow.
*Started getting backfire on deceleration this Spring (2015), read through the posts here on VTX Cafe which pointed to lean A/F mixture. Picked up permanent mount A/F screw from Glens VTX Garage in June. Problem solved!
*Two days ago, was changing lanes and bike hesitated and then backfired while trying to accelerate. Worrying, but it drove the rest of the way home OK.
*Yesterday, hesitation and backfiring started getting worse, couldn't accelerate to highway speeds. Barely made it to work (10 miles from home) and almost didn't make it home, spitting and sputtering the entire way on back roads.

I have more than 1/4 tank of fuel, tried reserve position just in case, no change.
Tried riching out the A/F 1/4 turn, didn't help.
Tried driving with the choke out a bit, no change.
Starts and idles just fine, only have issue when accelerating.

Any ideas? It's got to be a fuel flow problem, right? I'm confident I have the basic skills to pull the carb and clean it up, but due to how sudden this was, I'm completely baffled as to what the root cause could be.