I must admit, that I have not seen any adverse effects..

and trying to cast lightning bolt in the middle of casting does not trigger a GCD... at least not noticeably so... the GCD on the client is reset if the spell is unable to be cast due to it being triggered too early.

All that is being penalised for an early press is the round trip time to the server... so you lose 200ms... lets face it as boxers we aren't shaving it that fine due to sync issues etc.