Ditto,
I understand your frustration over what the manufacturer's state in their documentation. But you also have to take into account that a lot of people in the RC world, and here on RCU, will over power a plane to suit their style of flying. So who's right? Both of them are? And both are wrong too. What you need to do is find out how you are going to fly, and then you can start figuring out where you power an aircraft. I consider myself a purely sport flyer. I like to perform aerobatic maneuvers with my planes, but I don't need to be able to do 3D. For myself I have always used the upper end of the manufacturer's recommendations and I have never been disappointed with a plane yet. While my choices suit my style of flying, they would be totally out of line for a 3D flyer.

It's been my experience that the recommendations by the manufacturers are there for the average pilot out there. I don't think that have under estimated any recommendations. For the average pilots the recommended motor will usually be just fine. But like I said above, you have to learn to adjust according to your flying styles.

Hope this helps

Ken