I have in my (omg I just realise how long!) 11 or so years of infrequent paintball seen 2 lenses crack from impacts. One was a recball sites single lense and it wasnt even a close shot. Those gogs take punishment week in week out, but the velocity of punter markers on sites are usually lower than 270fps in my experience. Of course site lenses I know rarely get changed.
The other was a thermal lense which was less than a few months old, at a tourny, and lets just say the flimsy inner layer did help, but he still got shell and paint in his eye. We are unsure if the gun that shot him was hot, instinct says very...but either way, his lense didnt hack it and a new one might of.