I would say it depends on how bad the flex is. My transom had some flex in it but no cracks. The PO had put an aluminum plate inside the splashwell, almost all the way across. I added a huge plate outside just for peace of mind- I figure I can get at least 3 seasons out of it (found the source of water intrusion and sealed it). I have NO FLEX at all and the thing is solid as a rock now.
I painted and trimmed the plate outside to match the hull
