Ever concider Corian? Commonly used for countertops, people are now making signs out of it, I make the parts in the picture attached. The pictures do it no justice, the color is very rich. Corian is also free machining, the chips won't melt, they actually turn to dust if you pick up a hand full and rub your hands together. You can cut it as fast as your machine will go or otherwise limited to spindle speed.
Lexan is easy to cut, but the coolant will mess with the material and the chips tend to melt and load up.