OK for all you machinist/engineering types out there here is a question:
Is a drilled and tapped hole (bolt) into fiberglass more resistant to pullout than a correctly pilot drilled sheet metal screw of the same diameter?
Glass fibre never holds a fastener tightly for long and will wear away with even minute movement and a fine thread will leave very little "meat" for it to be strong in GRP (OK in metal). So a fine thread on a bolt will let go first, a course thread on a bolt will let go next, a course self drilling screw will let go third and a through-bolt won't let go at all.
I know what my choice would be.
BTW, if it's a light-usage fitting in reasonably thin GRP, the better fastener IMO would be a pop rivet of suitable size, as long as the end of the rivet goes all the way out the other side of the skin.
But still, a doubler plate on the other side would be my choice.