A more open filter generally causes your engine to run slightly leaner if anything.
Is the engine 4 stroking and sputtering under full throttle?
The grey color of your plug concerns me, as this could be a sign that your cylinder is shedding it's chrome plating. You also have a lot of speckles on your plug's interior metal section surrounding the insulator, which could be bits of engine cylinder liner.
Also there's a lot of baked on oils on the threads of your sparkplug, are you torquing the plug down firmly, and are you still running 16:1 fuel oil ratio?
I read about 4 stroking few times, but I am not entirely sure what that means. It doesn't sputter, but rather feels like it loses power, unless I gradually go full throttle on flat ground.
I was having a different issue after cleaning the carb few weeks back (I bent the tang down without realising what I did) and just completely lost power at full throttle, put it back up again now. At the time, before I realised what was the cause I added more oil to my gas can mix to turn it into 20:1, was running 24:1 previously, it did improve the issue, but I was getting at the wrong thing at the time. I am open to a recommended mix, currently got synthetic oil as well, decided to use that to make sure everything is lubricated well and the engine last, getting metal shavings instead is the last thing I had in my mind lol
Regarding the torque, not having a torque wrench, go easy on the twisting in fear of stripping threads with it being aluminium and all. Still working on the "doing it by feel".
Edit
Should add I have a banana pipe without a muffler, fell off, will add after solving this or just give it a go with muffler putty tomorrow. Didn't notice any power increase, but engine runs much smoother overall, minus the engine going back to idle has been hit or miss lately (exhaust gasket has no leak, had that issue with the old exhaust, already checked)