Switching from a 69% efficient stove to a 78% efficient stove will bring about a notable reduction in wood consumption. Switching from a 74% efficient stove to a 78% efficient stove will bring about a trivial savings. Where the BK may save more wood is where the thermostatic operation does a better job of regulating the burn than the 74% efficient stove is being operated sub-optimally. This is not an unusual case. It's easy to get distracted at times with 2-3 fires a day over many months and to turn down the air too late. If there is no flue thermometer, it's very easy to happen.
Highbeam, correct if I am wrong, but I recall regular reports of 800º stack temps with the ineffient Heritage. That was wasting a lot of heat up the stack. Now, with the Princess the stack temps are less than half, which equates directly to fuel savings. However, when pushed harder for heat, we get consistent reports of burn times and stack temps more close to a well run, efficient non-cat. For me that was the deciding factor to stay with the T6. I'm retired and can stay on top of stove and flue temps. Yes, I may burn a bit more wood, but that gets made up for by running the heat pump during milder weather. This is more efficient, less costly and much cleaner than the wood stove. Also, I am admittedly biased toward KISS design in a stove. Less tech means less to maintain and less to go wrong and lower operational costs over time. That said, I can appreciate why a cat stove is a good solution for some people. If our house was better insulated with less window area and we only had an oil or propane furnace, a cat stove would look more attractive.