But aren't people posting videos where they are out of the AOE zone for much more than 0.3 seconds? I don't think it's the only problem, otherwise every player would be affected to the same extent, which (despite what many want to believe) is not the case.

Also, if adding time to the cast bar to account for lag is the only available solution, I would say please don't bother. Players that are not experiencing issue will then have even more time to dodge AOEs. Imagine how unfair that will be in PVP.