Darn, I just checked the calculator for my benq w1070 and found out how right you are about the zoom.
At max zoom, 1.3, I get 18 fl at 138 inch 16:9 screen size (181 inches throw distance) without the lens.
At min zoom, 1.0, I get 14 fl at 138 inch 16:9 screen size (138 inches throw distance) without the lens.
I moved my projector back by a few feet so that I could use the minimum zoom with my VC lens, to avoid vignetting. Now I find out that I'm not gaining any brightness by using a lens, however the image is less crisp and the entire setup more complicated, basically for nothing.
I think H-E lenses make much more sense if you can still keep your max zoom setting. Maybe I can try to use somewhere between 1.0 and 1.3 to get right to the limit of the vignetting zone, and get some kind of brightness gain. I will have to calculate it, or maybe sell my lens. I was thinking of getting my next projector with a greater throw ratio, so that I could get more of a benefit, but I like the idea of the projector at the back of the room and this makes that a low-performance option. I really like more brightness for pop, especially in 3D, in fact that's why I bought my projector.
I wonder if I replace the lens on my w1070 if I can get a longer throw ratio with less brightness loss, and perhaps crisper optics in the bargain. Then maybe I can some kind of benefit to using my VC lens because as of right now, when used with a short throw projector like the w1070 that requires me to use the minimum zoom to avoid vignetting, it's basically a downgrade.