If you restrict the number of HRPs, imho, you should also restrict the number of cargo bays. If you do either, it makes the ship useless for what it's intended to do... therefore I say keep them as they are.
A short while ago, HRP's offered less armor than they do now. The end result was a hull-tanked Cobra would offer less armor than a Cobra now with a single HRP.
Yet combat-Cobras were not "useless" because of it. Reality disproves your claim.
Lowering a theoretical maximum doesn't magically make a ship "useless", because the only point in the high theoretical maximum in the first place is because everyone else has a high max too.