I have the same problem with mine, I went out for a rip with my MBRP can on and it said muffler across the screen as well. With that octane test kit it fixed it right up or do you get the muffler display anymore?
Was at the dealers on the 22nd and they were talking on the phones about the issue. Didn't hear about a recall tho'. It was the preliminary phone call apparently.
Blair got his 2013 flashed for the 2014 upgrades and we rode on Saturday and no codes came up from TheThing can.