Yep, you're right, but it's no use having a 8 ton bomb load when
a) you can't ensure a sufficient number of bombers reaching the target due to survivability issues
b) half of the time these bombs are dropped on empty countryside and
c) you lose a few hundred of hard to train aircrew each night, while the Mossie only has a crew of 2-3
Of course, during the late stages of the war that accuracy improved and air superiority was in allied hands we could argue that RAF night heavies could at last do a proper job so we can't really discount them totally. The reason is simple, you can't develop new heavy bombers in the span of 1-2 years during wartime.
So, while it might have been more effective to use Mossies until advances in navigation, accuracy and survivability were made for the heavies, we could also say that if no 4 engined heavies were around for the early part of the war then there would be no reason to improve and refine them or the tactics they used, so in the end there probably would be no heavies at all. Interesting conundrum this one
