ATC and High Temperature Cooks

Question for the members - how are ATC systems for "high" temperature cooks? I'm referring to 325-350. I have a WSM 18.5 and have found that when cooking turkey or chicken I need to prop the door open to get the temps in that range. It tends to overshoot, I close the door, drops back too far, etc. So far I haven't found the right balancing act on the BBQ tongs to get the desired temp. Since I'm toying with the idea of getting an ATC I'm wondering if it'd solve this problem. The concern would be that the fan wouldn't be able to provide enough air to run at that temp and I'd still have to prop the door open even with an ATC.

For Thanksgiving turkey, I was able to maintain 350 for about 45 minutes, then had to drop down to about 325 when I noticed it'd started to cycle (waves in the graph). (I could've just adjusted the pid settings, but didn't want to try to figure that out.) It held really well.

Then a few weeks later for a family reunion I was planning to cook another turkey and while setting up my heatermeter, I noticed I pulled the wire loose from the fan. I didn't have time to resolder (and upon later inspection noticed I wouldn't have been able to anyway), so I had to drop back to manual control. I started with a few more briquettes and opened all the vents, top & bottom. I ran the HM w/o the fan, just so I could monitor temps and I was able to run about 320 or so, ...with the door shut. I could've started more briquettes or left the ones I started a little longer on the unlit coals before stacking the wsm if I NEEDED to get 350.

This proves that an ATC is nice to have, but (in my opinion) should only be a purchase made after you've mastered your smoker. ("Master" here might be a bit strong...)