Hot but Stable - Clutching at Straws
I say get rid of the filter
Putting in a hotter stat to run cooler is BS
Pumping water "too fast" is BS
Suspect the gauge
Eventually, when you can afford it, a new, larger rad can't hurt
If it's stable temp and is not puking out the overflow, I'm not sure I'd worry about it
What is your ambient temps, and how dry is it there? (Hot dry air has less "cooling ability"), IE, lower "specific heat"