http://www.bangkokair.com/pages/view/check-in-through

Mentions through checking of baggage when leaving Samui but does not mention when travelling to Samui - I suspect normal international-domestic rules then apply i.e. pick up bag at BKK and check it back in for the domestic flight.

http://www.bangkokair.com/pages/view/check-in-airport

Domestic cutoff is 40 minutes. So you have 45 minutes to clear immigration, pick up your bag, clear customs and walk to PG in island F http://www.bangkokair.com/pages/view/check-in-airport

I really reckon it depends how bad the immigration queue is. If no queue 45 mins is a breeze but some times it's not.