It would help to know how wide your foot is when measure on a Brannock, but still this will be a difficult question to answer. I'll try a few things.
You say you have always skied a high performance boot, yet you are 200lbs skiing an 80 flex. Doesn't add up.
Since the sides of your feet hurt and not the top it may not be an overbuckling issue although to hold you securely in that boot you may be overbuckling.
I'm more inclined to think the medial malleolus and navicular are hitting, moving your foot laterally and that is causing the pain. Footbeds may help but if I am correct you'll get more relief from punching in the ankle area.
Suggest you also start the search for stiffer, taller lower volume boot.