I see exactly what you are seeing, but also the ias of the b26 is so low that manifold pressure jumps more than anything else. I think my ias was little over 200. if I move it back 5 or 10% it manifold does go down to 50, but if I move it slower back and forth it the manifold pressure moves a little slower.
I mostly use the b26 to sink cv's. at around 6 to 8k ias is more like 282 once I level out and attained full speed. above 16 not sure, if it get faster than 204. have never tried it. I tested it at 19k or so. flew for perhaps 15 minutes and dont recall ias being any higher than 204 or so. that was with 75% fuel and 4 1k bombs, flew about 4 or 5 sectors, dont remember exactly, think it took me 13, maybe 14 minutes to get it to 17k, after that lost track of time.
think is similar to the bostons with full fuel, I'll climb to 20 or 25k. on the ground it says 34 minutes worth of fuel, once I get to altitude and level out it says I got something like 32 minutes worth of fuel at full speed.
semp