The MPD will not effect the baro, however it will effect the overfire draft. Below you can see my quote from the MPD thread.
"What I did was check the draft at three separate places. First at the stovepipe, prior to the baro. Next, at the draft door on the combustion fan. And lastly, I checked the overfire draft. I tested each point with and without the draft blower going.
Open or closed, the MPD had no impact on the draft feeding the fire. This was true with and without the draft blower.
Open or closed, the MPD had no impact on the stovepipe draft with the exception of when the blower ran. Open, the draft would climb very high, very quickly. Closed, the draft would bump up slightly, stay fairly steady and rise very slowly over time. This would confirm what I thought earlier, that the boiler will run the draft motor longer at a lower load as it should.
Now comes the big surprise, the overfire draft. When testing it with the MPD open or closed it had no impact on the overfire draft, which I thought was rather odd. Then the blower came on. BINGO! the draft went neutral instantly. I watched it for a while and noticed that the draft was going positive as the blower continued to fan the fire up.
I don't know about all the other chimney/solid fuel units out there, but mine proves if you don't have an airtight unit with a decent draft, you can kill yourself. I don't want that to happen to anybody, so check everything and be safe. Learn as much as you can and get the tools you need to run these things right. Burning coal is supposed to be a warmming experience, not one that leaves you stone cold."