PaZ71

Hi everyone, new guy here. Trying to learn a thing or two lol

So I have a 2008 Silverado 5.3 that has had the following done this Spring. The work was not done by myself.
TSP-212/218 111LSA Low Lift
TSP AFM delete
Yellow LS6 Springs
Black Bear tune
Valve Job (He said there were a few burnt valves)

Last October it began to misfire going up hill, which ended up being a bad lifter on Cylinder 3 if I remember correctly.
I drove the truck up until January like this, only short trips when needed. And you could smell un-burnt fuel every time it was run.

Fast forward to now after the cam swap and tune.
The check engine light (P0300 code) and traction control light will flash when the motor starts turning higher rpms. But the higher the gear, the lower the rpms to misfire. Its about 4200 rpm in 1st but 3800 rpm in 3rd when it begins to misfire. It feels like there is a rag in the intake when this happens. It almost seems to quit revving and I should add it feels like is down on power before it begins to misfire.
I will say it did not misfire under load right away but it did feel kind of doggy. For example there is a hill by my house and I had gone up it a few times no problem and then boom misfire and down to 20mph.

I sent Justin at Black Bear a data log and he suggested that I have very bad valve float or a possible MAF issue.
I did replace a post CAT 02 sensor on the drivers side bank as I was getting a P2271 Code along with the P0300.

So I am wondering (with limited knowledge) if the cats could have been damaged or plugged from driving the truck with a misfire and if there is any chance that the cats could cause a problem that would be similar to the valves floating? LS6 springs seem to be commonly used on low lift truck cam swaps with no issues but I also have terrible luck lol

Any help is appreciated!