I suspect it will turn out to be an issue, but maybe not; might as well give it a bit of real world time and see the frequency and severity of actual issues it causes (or doesn't). Would have a better idea how to resolve it then, too.
I've seen players put literally 500 turrets under one set of overlapping forcefields, and then drive back 12,000 incoming ships without any mobile ships to assist.
I don't really like it, seems hack-y and artificial, but just to throw it out there: turrets get the 25% redux and everything else doesn't? If that scenario is the major concern that could work...
On the defensive side, there is NO reason to have piles of forcefields on your side of the wormhole on top of the wormhole. ... The problem is when the AI ships start inside the forcefield to begin with, and then don't have a reason to leave. That's something that should be avoidable in most circumstances.
Agreed, but it's bound to come up. This one wacky example I'm playing with now aside, I routinely build FFs adjacent to wormholes/com center/bonus buildings... occasionally those buildings are around hostile wormholes, and even when they're not, you'll have baddies popping through during CPA mayhem. If a bad guy does come through dead in the center of your FF, and makes your FF top target, sometimes you'll be able to handle it fine, and sometimes all you'll have is fighters or whatever and you'll kick yourself if there's the 75% penalty. That, and then you have your general mobs of guys and FRD chaos... overall there's often going to be a small fraction of fire that ends up getting reduced. Well, that's my prediction.
But like you say, may be a non-issue at the end of the day.
Random side note; What units can actually deal 1m+ damage / have sufficient apen to deal more than 20% damage to hardened shields?
Go to reference tab, pick something like Wormhole Guard Post, sort by Shot Damage Them > Me, turn off filters. Looks like several dozen do 1m+, but mostly Spire (storyline?) stuff. There are a few real-world examples though, and like Keith said, you could get Armor Rotters or something which could take them down a peg...