When my bike was in the shop getting a new head tube I was riding on a borrowed CAAD5 frame with a dented seatstay which didn't seem to have any effect at all on the performance of the bike. (it was also a bit harsher than the Ti frame I'm used to but surprisingly not as much as I'd thought it would be)

When I was in the shop picking up my frame someone had brought in a Carbon Cervelo that appeared to have had a very similar crash to what dented the C'dale. Unfortunately for them though, the seatstay was broken in half instead of dented and useable.