Boost is going to be relevant to the amount of restriction in the inlet and outlet paths of the engine. While they may make 8psi on a 5.4L lightning, you may come up with a totally different figure with a 302/351. Higher boost pressure may just indicate less restriction in the intake path, or more restriction in the exhaust path. Remember, more boost equals more heat, so overdriving it too much may actually be detrimental to power output.
I’ve seen blown cars with intake and exhaust mods actually loose a couple of PSI, yet continue to make more power. Lower boost pressure doesn't necessarily mean that they’re going to be making any less power. It just means that their engines are breathing more efficiently with less restriction, while creating less heat and in the process.