I just bought a pair of 661 Evo XC kneepads. They are a thinner version of the Evbo pads using D3O as well. They are very thin(about 7mm D3o) and flexible. They are precurved and did not feel like they would inhibit movement at all. They are perforated though out the entire D3O pad so they should breathe quite well. If they do bother you, they can be taken off with removing shoes etc. because the sides open up completely.
They are very thin. They offer minimal protection, think of this as a rash and bruise prevention, not full on kneepads in the usual bike/skate sense. I think they will be a great option for XC style riding(as they name says ;-) whith shorts, giving you a bit more protection than long pants with better venting.
The weight is 316 g (11oz) of my size S/M pair.
They would do well as kneewarmers because they are so comfortable and breathable, much better than others. Also because you can take them on and off easy. I think the elbow version of these would be great, as you tend not to land so hard on your elbows.