Here is my question for you experts, or knowledgeable novices. If a bow is tillered for a specific draw length say 28 and is drawn at a shorter draw weight will it decrease the efficiency of the limb? I know it will lower the draw weight, so my question really is would it make the limb less efficient if the same limb (core, lams etc.) had been tillered for the final weight at that draw length. So tillered for 55 at 28" draws 50# at 26" say. Would the same limb tillered for 50# at 26" be more efficient?

On glass bows, maybe not so much tillering as design.  Some bowyers adjust the taper rate of lams, riser length, power wedges and tip wedges to get  maximum efficiency at the desired draw length.  Others don't.  

Depending on the design, with average bows that are being drawn 26" - 29", a bow that is designed well at 28" works good.  You could spend a lot of time maximizing it for a 26" draw for a very small gain.

The real answer is probably a little different for each design and each bowyers approach.

I think cast would decrease if using the same arrow and same bow. Hence, a loss in efficiency?