Hi Guest, Not had any dogs with a hip replacement, but one of our cats was hit by a car when she was 18 months old and we were advised that the best course of action for her was a hip replacement. Being a farm cat she lives a pretty active life and initially I was worried that she would be a bit slower after the op. therefore perhaps putting her at further risk from traffic, but the vet convinced me that this wouldn't be the case.
Anyway, she is now 11 and although we notice ourselves that she is a bit uneven when walking, people that don't know her history, don't seem to notice at all.
Obviously, this was just one hip rather than both, and a cat doesn't carry as much weight as a dog but I certainly don't regret getting it done.
Good luck with your cavalier, hope all goes well.