The official party line is as follows (if anyone has a more recent quote, please chime in!)
Found here: http://www.swtor.com/community/showt...=705142&page=2
All that being said, there is also a slight delay between your PC and the server, so sometimes it may appear to you that you engaged a missile breaking ability before the missile has impacted, when in fact the server has already registered the hit. This is pretty unavoidable, so my best advice is don't wait quote as long to Barrel Roll
Seems to be a few things wrong with this.
#1. Upgraded cluster missiles will still hit during barrel roll if you waited until the missile was launched to use the barrel roll, because the second missile will hit, but not the first. this results in a hit for reduced damage, but still a hit. I'm waiting on Bioware to decide if this is intended or not.
#2. If you use distortion field to break a missile lock before a missile has launched, it'll break the lock, but they will immediately be able to lock onto you again, it does not break the lock for the full 6s (or 3s that it used to be before). If you use it after a missile launches, it will not avoid the missile. This I believe is bugged.