I picked up one of those cheap toy projectors. They great thing about them is that they use standard halogen bulbs. So I went to Home Depot and got the brightest version of the bulb I could find which was double the original lumens.
I found that the limiting factor was the heat. After a few minutes a black circle started to appear on the image. It was the bulb melting the LCD panel. I had to put a little desk fan next to it to keep the air moving sufficiently to keep it cool.
These little 75 lumen projectors with a low resolution are likely running into the same problem. Obviously much brighter LEDs exist but there's not enough space to properly cool them.
The resolution is good enough for a smart board which only can handle 480 vertical lines anyway. I opted to get a the $320 proper projector with a 1024x768 resolution. Smartboards are easily replaced with a much simpler HD document camera.