Three Things:
First: You´re trying to put 50% more power. The hull might be capable for using 15hp, but if your hull was for 10hp id say be careful... but (read next)
Second: As you are trying to put 50% more hp, and the length of the aluminum makes the push even harder for the transom.
Third: The force applied at the braces, and how you pass the load from the lower angle to the upper one, its way incorrect. Then, the joint between the two angles, is too short, making it the perfect spot to get bent when turning fast or decelerating when turning.
If you ever hit something underwater, you might end up with a bent motor holder, the motor making a hole in transom, or the motor running furiously everywhere, not to mention getting over you.
With that design, you are effectively trying to break your transom with more power and more leverage. You're trying to end up in the water. Not judging you, but I really think thats not very smart.
Brace properly the transom side to side and it might be safer, use thicker square tubing, put a brace from side to side at the top of the transom and in the lower part inside the hull. The effect of the motor is two things: Pushing at the top of the transom and pulling the lower part.
My honest .02 c