I live at 5,883' or so and cook on the mini just about every weekend. I have no problem getting to 375 during braising, and often get it under 250 easily. I use the smokey joe gold as a base too. It's favorite temp seems to be 250 with a 1/3 lit coals, top exhaust wide open and bottom vents completely closed - it will hold that steady for 3-4 hours.
My guess is that it's your diffuser if you are already using 1/4-1/3 chimney of lit coals and can't get it above 200. Are you using a clay saucer or something to that effect? Is your bottom cut out or do you have holes?
Mine has the bottom cut out, a charcoal grate sits on the lip of the steamer insert, and on top of that rests a 9" aluminum pie pan.
***that's also quite a bit of food on there at the same time - could also be restricting airflow, but I'm almost positive it's your diffuser being too big and/or diffusing too much
Mini WSM w/SJG, 22.5" OTS, Smokey Joe Silver, Weber Q220 (the fiancée's)