One of my cats is pretty fucken heavy, (although he's quick and plays a lot, he's just strong and heavy) and used to try and climb the screen door, one day he was pouncing at something he saw outside and took the whole thing down heheh...

My previous cat though had jumped down from the second floor balcony at one point when he was being watched while on vacation by a family member, I guess he wanted to get out at their house and felt trapped or something (came back the next day) but he was ok after jumping out, even when landing on a driveway.

I know you don't want your cat falling out the window by accident! But I think they would be ok, they go in trees and all that...