I actually observe the AI doing this, placing mines around key structures. Would 5-7 mines fields be reasonable? Has anyone tried something like this in a mod?
The unfortunate reality is, even if I use all 150 mines, or attack a grav well with that amount... I really don't think they significantly influence the fight. Maybe the mines tear up some heavy cruisers, but that's it. The fact most people hate mines because of how annoying they are to remove, not because of any tactical impact, says something.
More to the point, I think I reexamining mine effectiveness and removal would be of more use than tweaking grav well limits. Not a terrible idea, though.