Your package is at 66% of the power rating which isn't that bad.
The Xtreme 18 is only a foot longer like you say but it is a much bigger boat. The hull is 4 inches deeper, the beam is 8 inches wider, the hull is 200 pounds heavier and the capacity is 350 pounds heavier. The HP rating is double. I stand by my original assessment of the 50hp engine on that 18 Xtreme.
Percentage wise, that package is comparable to your boat with a 25 on it, which is currently half the size as what you have.