I'd be very surprised if the KA6 boiler can make it's rated btu with the much smaller flat grate stoker. I don't think he CAN order a KA6 with a flat grate stoker unit..
I think the difference in the grate performance is that the flat grate stoker will have a deeper firebed.. so the coal should burn more completely.. Very similar to an underfeed stoker burner or the Harman style 'push up' style stoker..
I did an experiment with a Leisure Line stoker... I put a 'speed bump' at the end of the grate, it's effect was to slow down the ash as it was pushed down the slanted grate.. The coal bed became about twice as deep, maybe 2.5" instead of 1 or 1.5" deep before.. It seemed that I had a more complete burn of my coal,,, and had less unburnt pieces.. but my experiment was short, I changed to a different coal, so my 'before and after' data was no longer valid.