The cover bonuses are fine.
Bear in mind that cover bonuses are supposed to reflect a combat situation in which people are jockeying for position, popping out of cover to take shots of their own, etc. So it isn't necessarily covering 50% of the body for every moment of the entire round.
The initiative system creates the impression that the person is just hiding behind cover, waiting for his turn to move, but that's just an abstraction that we use to model a less grainy reality.
There's also a game design rationale: if you make the cover bonuses too large, it could create incentives for characters to behave in a way that defies the genre conventions which Pathfinder is trying to represent.
I think it's partially a matter of which parts of the body are covered. A breast plate covers (for the sake of argument) less than 50% of the body, but it's covering many of the best and most convenient places to stab someone, and someone wearing it can manuever themselves so that it intercepts blows that would otherwise find vulnerable targets. Cover isn't necessarily doing that. A "hit" in pathfinder doesn't represent merely contacting any part of the target with your attack; it's landing an attack that means something.
If for whatever reason you wanted to, you could probably increase bonuses from cover. I doubt it would break anything to bump it up a point or two if it's that important to you.
For the most part, you can only count bonuses as how they reflect to a d20. For instance, stating +8 is nothing, can be real true, or totally not true.
If I needed to roll a 12 and your +8 made it a 20' I only have a 5% chance to hit you. If I needed to roll a 20, your +8 to AC doesn't change anything.
So, you are forced to keep it within the d20. Anything that cuts off 4 levels of fighter BAB by just standing in the right spot seems real good to me! As stated before, it is about you and your attacker trying to get good position.
Get a few friends and make 2 squares in your back yard, all 5'x5' in a line to total 5'x10'. Now stand In opposite corners, each with a yard stick. You will see that you can barely touch the sticks together, let alone touch each other with them. You will also see that you can easily fit 5-6 people in each square.
Like stated, combat is abstract.
Now, about gunslinger, it is not core and cannot be compared to cover. I agree that it is not my cup of tea, so I don't use it. But to others, it is. That is how they want to play, and who cares? It is not an MMO where we have to all use all the rules. It is not like you are going to get PVPed by a gunslinger in your house if you don't want to play that way!