I am very sceptical about this too, that longer mast needs much more power, I have never felt any difference whatsoever in waterstart or low end.
Riding with 83 and 91.5 and 100 cm masts, dont notice any difference in being more or less difficult or anything.
Are you sure it isnt just because you assume you need more power, it feels like that and becomes an issue?
Just like we feel we can start a lot earlier in a summer shorty, than a full 3/2 suit, because it feels so much lighter and better, where, if you look at it, the weight difference is almost nothing, compared to your weight.
There might be a fraction more drag theoretically, with a longer submerged mast before up and foiling, but compared to the drag from the board before up and foiling, and the wing/fuselage/stab, I think it is close to zero extra drag.
Meaning, I dont use the short mast anymore, if wind is marginal and a bit sideshore, only in dead onshore where it is the only option.
But the 91.5 cm is a great allrounder for everything though, my choice if I only were to pick one.