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That sounds normal to me. I only have a large...not XL...but the principles are the same. Temperature is controlled by air flow. How open the bottom vent is doesn't matter if the top is restricted. i.e. if you're restricting output airflow (at the top), the amount of opening at the bottom (input) doesn't really matter. If you want higher than somewhere around 300F, you need to not only open the petals, but also slide the daisy wheel open to some degree.
I've had no problems with the non-greenwise chickens from Publix. Can't remember the brand (Perdue?)...but I generally look for the biggest they have and land in the high 7 to low 9lb range. Indirect at 450F works here. Cooking veggies in the drip pan adds noticeably to the flavor of the bird. I skipped that part once, and we had a fairly tasteless bird.
The biggest downside to the One is the non-adjustable mounting bracket depth. Your vessel has to be deep enough to accommodate it. One other annoyance...a brown-out will stop your cook on a One. I suspect it will on the newer one, but haven't found out yet.