Yes, but even if you factor in the rate-of-fire for the mini-gun, the Howitzer would do more damage.
If you did that with the mini-gun and shotgun, the mini-gun would do more damage. (Well, I'd assume so.)
Anyway, we don't want to stray too much off-topic.

Silliness aside, isn't the Maxim meant to deal extreme hull damage per hit anyway considering it does close to no shield damage?

It's supposed to be an anti-hull weapon, so it does damage primarily to hulls. But since it has a high ROF, each individual slug does moderate damage to hulls, and almost no damage to shields, like you said.