Because of the open rules about spying this time round, if you didn't have a limit, then someone with an obviously slow first boat would simply build an exact replica of the fatest boat for boat 2, no point trying to evolve what you've got if you can just leap to the head of the class.Why would they restrict the amount of changes that can take place on the hull? Is that unusual?
It seems like that rule is intended to lock a team into their initial design decisions and that is the reason for the "cup is already won" comments. It is as if they didn't want people to be able to incorporate one teams design ideas into another teams hull.
Still 25% of the surface area is actually a pretty huge amount of change when you think about it, theoretically if you leave the top half completely alone, then you can change approximately 50% of the underwater half!