I'm looking at buying a set of 32P Bravo I props that have the hubs cut off. What effect does this have on performance - planing speed, top end? What's the purpose of removing the hubs?

By hub you might mean diffuser ring; the flared or belled end of the prop hub. Cutting it off is usually done to decrease transom lift a bit at speed.

It can make getting plane more difficult. It also can add 50-100rpm at WOT

Exactly.
If you blow out getting on plane, leave it on.
But if planning is easy and you have plenty of stern lift, removing it will decrease drag slightly and you may pick up 100 or probably less rpm which can translate into speed.

Brett

Not sure if it makes a difference here, but are the 30 and 32 Bravo props also a smaller diameter?

The Bravo 1 family has verying diameters. The family is advertised at 15.25" diameter but the 34 and 36 are the only ones that are that large. The rest of the family is 15" diameter or less.

Brett