let me start off by saying, your title is confusing me, cause by your post, it appears you've gone back to wood. 2nd, my opinions here are coming from less than a yrs worth of burning coal, so don't go mortgaging the house on what i say. However this site has provided me with not only entertainment, but helpful advice.
I too just installed an mpd to my 6" black pipe behind my stove in an attempt to try and slow down the burn in my Surdiac, which may allow me to burn "longer" before my grates clog up with ash. I've been experimenting with angles and openings of the intake damper, (a round "flapper" valve, much like a toilet tank), and the mpd. My mpd is also "cocked" to one side of the 6" pipe, and does have holes in the middle. After my stove has been fired, and the mpd which starts wide open, the 1st time I closed my mpd to approx. 45*, it appeared the draft became accelerated and the coals burned hotter. Wasn't sure if this was the case, but in time i've learned that by closing it to this setting "and" turning down my intake damper, the stove will slow down. (as it is doing right now.) I've tried a lesser position, (approx 15-20*), but don't think it's that sensitive. As for closing the mpd more than 45, for me, don't think it'll be beneficial. think it would be too drastic.(But who knows? might try it some time) I'm also trying to get through the nite without losing the burn, and with my stove and set-up is difficult. The past 2 nites, I've put my damper to a 15* angle, and have woken approx 5-6 hrs later with the stove "relatively" warm, and have been able to refire the coals. I'm afraid to choke it off to 45* at nite, fearing it'll be too slow, and I'll be worse off in the AM. However, I am going to try this in a few days, when our outside temps will be warming up, and i want it to go out. That being said, good luck with yours.