Actually, a flashlight is very telling. I used a sample of the HP, superimposed on a sample of Vutec Silverstar. I shined a flashlight on both and moved them around. Make sure you orient the screen materials the correct way. On the SS, for example, you have to make sure you have up and down oriented properly. For the HP, its doesn't matter. Anyway, you'll be able to tell relative brightness this way. As you move the flashlight close to your eyes, the HP will achieve its greatest brightness. The SS will be brightest as the flashlight is raised above eye level, shining down on the material. Moving the materials together, I could see the HP and the SS change their relative brightness. It's also a good way to check for uniformity, having the two materials superimposed. That's how I was able to detect the SS sheen, whereas the HP maintained its uniformity across its entire surface.
From your sketches, I'd say the people in the outer seats are going to lose a great deal of brightness. If you can modify the seating to get them into a narrower viewing cone, you have a better chance of everyone seeing a bright image. A lot of this depends on your projector, too. If you have a bright projector, they might still be OK, but if your projector is dim (as most of the new 1080p projectors are), the brightness might not be acceptable for those outer seats.
Consider this, though - how often will you have guests who will sit there? Can you, for those occasions, crank the projector into a brighter mode (or open its iris), but use the projector at its optimal settings for most viewing, with the inner seats being used? If you can manage that, the benefits of the HP will outweigh the disadvantages. On those occasions when you'll have ambient lights on, you're not going to be getting the best image anyway. Why not enjoy the extra brightness for those vast majority of occasions when it'll just be you and a couple/three additional people?