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"