I've only seen a couple of jobs Jim's pans, but IIRC, they're a drop flue design right? I was thinking that 1.5" is pretty deep for a flue pan, but if that's also the level in the syrup pan, I suppose you can't go much less than that.

The first thing I'd try would be firing more often. You mention the fire dying down. When I was first running my 2x10 I did the same thing. Load the box full of wood, let it run for 10-12 minutes until it started slowing down, load the box full, repeat. I was getting about 30 GPH. After talking to another member here I used some tips I learned from him. Only open one door at a time and get it open, loaded, and shut in the minimum possible amount of time. Also, add only a few pieces at a time, but add wood every 3-4 minutes. The goal is to eliminate the fluctuations of heat from various stages of the fire. Instead you have a much more constant heat source. Typically I have to adjust my float as I get rolling. Once I'm up to temp and really cooking I have to increase the flow coming into the sap pan. Then when I start up again in the morning I'll reduce the flow because otherwise it'll want to run really deep and take forever to get going.

Beyond that, perhaps a baffle could be added to shield the float from the geysers. My 2x10 had something like that. Just a little cover that fit over the pipe that went into the float. Obviously not water tight.