Evolution wrote:a lithus is 25m +
a module will be 1m
see the difference...?
and yes, this module would be useful for transport bots.
Really, 6U more is worth 1M, a 10% increase? With all the penalties you suggest, speed, armor, demob, it wouldn't be worth using.
It would be useful on raiding bots, to hold a little extra loot though, and could be worth it for them, if it didn't have any speed issues other than the mass of the unit.
The only way I can see the cargo expander being useful on a sequar is if it allowed you enough space to run the Level 3 Beta missions in one go, but that would be +40U for a total of 100U. But that would make the level 3 payout inbalanced, and they would nerf it, making the mission worthless unless you had the module.
TLDR; There's nothing really wrong with the addition of a cargo module, but it wouldn't make the game any better.