I'm in favor of the diminishing caps, it makes the higher mark ships feel "special" and "elite". If needed they could be balanced as if they had the Mark I cap instead of a linear balance increase (So for example if a Mark I Fighter has a cap of 100, then instead of just being twice a powerful as a Mark I Fighter with a lower cap, the Mark II Fighter would have a capfleet strength equal to 200 Mark I Fighters, even if their capfleet is only 85). I think that would solve the diminishing returns problem while keeping the "few and elite" feeling of high mark ships. But it would also require a major rebalance, so...
Yeah, I'm gonna vote keep-as-is.