I know things... A lot of things.
Join Date: Feb 2015
Motorcycle: GSXR 750 K4
Sputtering indicates improper fueling and a glowing exhaust indicates a (very) lean fuel mixture, which would lead to raised exhaust gas temperatures. I find it puzzling that the glowing starts so far downstream of the exhaust valves. One explanation, would be incomplete burning of fuel, leading to unburnt fuel in the exhaust, which then ignites near the catalytic converter, because you've left a leak there, introducing oxyge. Whatever the case, fueling seems to be off, and you don't have any fault codes, so, given the actions that led to the problem, I'd suspect you've somehow messed up with the HO2 sensor, which now (possibly due to the aforementioned leak) sends an incorrect, but within spec signal to the ECU.
Try disconnecting the HO2 sensor, to see if the sputtering goes away. Do not operate the engine more than you need to under these conditions, that is, if you see the sputtering is still there with the sensor disconnected, do not wait for the exhaust to start glowing to shut the engine down.