Ian, did you ever get a manual for my 1930's IronFireman underfeed stoker??
Seriously though, do you have a manual for any underfeed stoker?? Like a Will-Burt?? My stoker unit has several settings for air both in the auger feed-tube, and above the auger in the retort, and it is pure guessing for me to adjust these settings.
Keyman, I think those flat deflector plates should stay in place, not only do the slow down the gasses heading up into the heat exchanger tubes, but the plates also reflect heat back down onto the coal bed, this helps make the coal fire more efficient.
I have several old trade magazines from the 30's-40's and there are ads for refractory panels that were mounted above the coal fire to reflect heat back onto the top of the fire, and to deflect and slow the exit of the burnt gasses.
So I'd leave the plates in place.
To get more heat from coal, you need more air, so I would remove the restrictor plate and try burning with full available air. The worst is that you reinstall the restrictor.
You may want to try a combustion blower, they definitely wake up a fire. The flames in my hand load fire went from 1-3" to 8-10" when the fan was running. But of course the coal burnt-up faster too.
Combustion fans are fairly reasonable in cost, mine is an AC-16, do a google search for that and I think you will find it, if not PM me and I'll try to find more info. I think they were about $58 each. I can't remember right now who I bought it from, sorry.
Experiment with larger/smaller size coal to get hotter/cooler fires. Stove size coal with a layer of nut over the top gave me the best results, but my firebox is not the same as yours, each design is different and likes different things.
Keeping the grate clean enough to get air to all the coal bed was an issue with my grate, I didn't design it very well for coalburning. Keep an eye on your coal bed for dark, or slow burning spots, you may have areas that are not burning at optimum.
Keeping your vertical heat exchanger tubes clean will help a lot with the efficiency of the boiler. Does the top of the exchanger come off so you can clean the tubes from top to bottom?? If so, set up a monthly schedule to run a wirebrush on a extension through the tubes. Keeping the insulating layer of ash to a minimum will really help.
Hope this helps, Greg L