This sounds a bit like a "Well, then I'll just neuter the systems and leave the command centers" mechanic.
How about making them not quite as "powerful" (how powerful are we talking, anyways), and instead let them spearhead small and fast fleet ship groups, that generally try to strike player territory while the player has a large fleet romping around deep in AI space.
This should either force the player to pull back, force the player to split his forces or at least leave a decent turret defense, rather than romp around with most of the fleet deep in AI territory until a bigger wave warning happens or he accidentally runs into a raid engine.
Note that this is mostly written from the perspective of someone who has only played against diff 7 AIs at the highest.
EDIT: Of course they shouldn't just throw themselves at well fortified chokepoints, either, but rather do some smart-ish cherrypicking of targets. If the player-area is too turtled in they could try to get the deep-strike fleet of the player instead. Whichever is less suicidal.