Besides the fact that the characters gain power from collecting/having them (and their power even being used for ships and machinery), each individual
Power Star is equally as strong as another in power.
As such, having a door with a requirement of X amount, or holding X amount is basically just addition.
In the case of holding them, they would linearly give the holder the power of each.
In the case of a door requiring a certain amount, given they're stated to be
magic (if you couldn't tell from all the magic stuff they do across the games)
in SM64 DS (sorry for the toad voice in advance lol, just read the text), it would linearly add layers of magical protection to the door.