I'm new to the board, so High, all!

With regard to the magnification of light:

Those dreaded "hotspots" come mostly when light is concentrated on a given point rather than diffused. If you don't focus the extreme concentration of the lens' cone of light you won't have hotspots and more than you can melt a crayon with the magnifying glass at the wrong distance.

Since an unfocused cone of light is still light, and since light is bent and concentrated as it passes through the lens, it should be possible to find a distance at which there would be an advantage to the increase of light available to the plant, yet no harmful heat effect.

Ever seen a lighthouse? Ever wonder how such a wonderfully intense and columnated beam of light can be produced from a candle? They use lenses called "fresnel lenses" to squeeze as many uselful lumens from a light source as possible. Photographers using the old sheet film cameras also use fresnel lenses. I have one for my 8x10 camera that is 8x10" and increases the light I can see by two stops (400%). A cheap experiment for those of you who care would be to get a fresnel from ebay for a few $$ and play with it. You might find that although it can't increase the number of lumens in your MH bulb, it can gather up a lot of the wasted light and bring more lumens to bear on your crop.

$0.02 . . .