We had a 22 Sisu in the shop that had the same issue with gunwales flexing while fighting a giant tuna.
To address this, we glassed in two knees that better tie the gunwale to the inside of the hull. In addition, you can also thru bolt the bottom of the rod holder through the side of the hull via a stainless reinforcement plate. Rich Oliver at R&R Fisheries in Ipswich sells these but I don't see them listed on his website. Give him a call..
http://randrfisheries.com/