You may be right to be concerned. My Destroyer started to stress crack at the top of the knees (Johnson 150). I fitted in a 25mm marine ply section against the transom between the knees. The whole area was ground back, the ply was bedded against 3 layers of csm, laminated heavily all over and down through the well, over the knees, along the transom and onto the deck. I paid particular attention to use diolen cloth between the layers of csm around the top of the knees. The diolen is a polyester fabric which is tough and very, very much stronger than glass cloth.
The cracking started within 12 months from new. After reinforcing, I owned the boat for 6 more years with no reoccurance.
While you're at it, remove that silly piece of tube used for the bailer. Radius the corners with the biggest curve you can manage and, while doing the strengthening, laminate right through the hole. By forming a large radius, you will at least double the flow capacity of the bailer.
If you want a job doing properly........etc.
You did ask.
Having said all this. From your pic, it looks as though there is a substantial extra thickness around the area at the top of the knees. Perhaps they now do it better.
I wouldn't even contemplate steel bracing. You've got to join it to the floor. What with, a couple of coach screws or a couple of bolts after you've made 2 new holes in the deck to gain acess for the nuts? Nah.