What is your honest opinion of the two planes if they had been required to swap missions? Lancs restricted to day light bombing for the entire war and B-17s allowed to bomb at night. Using the Grand Slam argument is silly because the B-17 could conceivably be modified to carry the Short, but Fat Man; 22 Kilo tonnes to 22,000 pounds. The lancaster's bomb bay was not nearly tall enough to carry that bomb load. So lets get off of the idea of the 33 or so Lancs that were specially modified to carry GS. It is the mission that dictates how many bombs you carry.
The first ~3,800 or so Lancasters could haul 14,000 pounds. Certainly more than the first ~1000 or so B-17s. The last ~4000 or so Lancs could carry 18,000 pounds, more than any of the last 8000+ B-17s. But none of that is applicable because it was Mission Planning that made both planes carry much less, so that the average for all missions was ~4,500 pounds for the B-17s and ~7,800 pounds for the Lancaster. If their rolls would have been reversed, the AVERAGE bomb loads would have also been reversed! Because load and Range are fungible, you can trade one for the other at will, over any but the shortest ranges. Because the B-17 was/is/and always will be more efficient aerodynamically, it will always win that trade off at any but the shortest ranges. Always! Because of that aerodynamic advantage it will always fly higher at any given weight. These two facts are not in dispute. You keep going back to the various placard numbers, but you have never explained why the Lancaster could carry 18,000 pounds but could only AVERAGE less than 8000 pounds per mission?