Honestly, I don't fully see the rationale behind the change you're proposing.
On smaller naval maps, games usually end before bulwarks can be produced in large numbers. Heck, the outcome is often decided long before T3 navy makes a full entrance onto the playing field. In these cases, I think the bulwark is pretty well balanced. It's not hard to get a few hover units or subs underneath the shield to kill it, especially considering the poor range and accuracy of the UEF T2 destroyer.
Unless you compensate with another buff, your proposed change will only make the underwhelming UEF T2 navy worse.
The Summit+bulwark spam issue you're talking about only becomes an issue on the larger naval maps, where the defender's advantage makes turtle-ish play with the bulwark more viable. These maps are played often, but I think it's important to keep the wider map pool in mind when proposing balance changes.
Maybe you could just nerf Bulwark shield stacking more? That might solve the issue without affecting T2 too much.