One can do some number tweaking and introduce some new ones utilizing non-used stats like what
@Paul.Ketcham mentions, i.e. I've added this one
but you can't really improve designers without improving the naval war first, and it leaves a whole lot to be desired.
One of particular bits MtG degraded is the XP allocation in ship construction. It's now totally binary, i.e. you either have some set XP (around 40-50) to make your desired setup, or you don't and wait till you get them. No variations are now possible, where you'd put your accumulated naval experience into better stats, so now Belgium ends up having the same ships as the UK across the channel (bar designer/spirits).
Now for my own mod I can sorta circumvent it by introducing extra modules, which take a flatout prohibitive time to research if unboosted by XP, but that's still very clumsy and inflates the research tab for no reason.
Designing is astonighingly poor in general, i.e. there are no options to consider 2, 3 or 4-barrel turrets, turret placement is irrelevant, armor is a single stat as if a ship is a solid ingot, engines are poor and even molded with hulls for research purposes, battleships are ahistorically prohibited from carrying torpedoes, fuel aboard isn't simulated and subsequently range is a dead-weight stat... and on the list goes.
Basically, all that MtG introduced with designing is some nice GFX, but even that one is spoilt by dubious ideas like one turret magically representing one and a half (or is it two?).
As others have said, -25% cost is +33% more ships. Oh how I wish it were -33% cost. That's +50% more ships! ;p
I recall us two having an opinion trade on this matter, but have you tried a simulation for two sides producing X IC of ships using two different designers and then putting them against each other? Even if Coastal Designer beats Raider/Atlantic ones [in vanilla], I doubt it's really 'blowing out of the water' type of an outcome - and tests surely depend a lot on initial inputs, i.e. if armor/piercing values are close, that +10% armor is something to cherish, and if piercing is way ahead boosting armor is of no use whatsoever (and neither losing -20% from Coastal one is a real penalty).