You could formulate a weighted objective function that summarizes the optimality of a design to a single number, then have the AI compare its current roster of designs with one the player has made. If the number is less (generally in optimisation the intent is to minimise an obj function), then the AI could start to use that design.
If you really wanted to go all out on this you could allow the AI to adapt its designs based on what the player is currently fielding (maybe based on intelligence - hey this could go both ways and give a use for the spy system!). That way the AI could learn how to make a decent fighter over time, and we wouldn't have the current 20/100:1 ratios for the entire war.
The weighted objective function wouldn't be perfect but it should be good enough for the AI to field air/tank/ship designs which can actually compete against the player. Choosing the weights needs expert judgement, which you as the devs should have in spades.
I'd also recommend you include a tickbox when the player is saving the design to opt out of the AI having automatic access.