Putting a 100 MN AB on a cruiser gives you slightly less speed then putting 10 MN MWD on it. So it is not unbalancing speed wise. It exchanges more speed tank IF you aren’t webbed, for sig bloom. And using that speed tank requires actually better piloting to get full effect because your alignment times with that 100 MN increase so much you have to actually plan what your movements will be and how to execute them.
Putting and XL AB on a Battleship would be the same. Get a bit less speed then 100 MN MWD, but exchange the sig bloom which is irrelevant because you are hitting battleships anyways, for far more alignment time on an already slow aligning ships.
All this would do is give players some additional fitting choices, not imablance anything in the slightest. Fitting the XL AB would be extra drain on CPU / PG, using it gives gigantic penalties to align time on ships that already have sig radius so big that additional sig bloom generated from MWD is completely irrelevant.
And if that freaks you out, then how do you feel about frigs using 10 MN ABs and moving at around 4k ?