The post has indeed switched gears.
Somehow we went from gaurenteed reduction of utility bills to merely gaurenteing the capacity and efficiency to be per the equipment specs + or - 10%.
Your right Norm, this is easy to do and not an unrealistic expectation. However, lower bills is misleading in many cases. For instance, if there is a room in the home that was poorly heated or cooled, and the contractor address that issue, he in effect has increased the amount of heating or cooling needed to satisfy the load based on the expectation. How do you show that as an energy savings? If the new system has a dehumidification cycle, how does that save money? It doesnt, its comfot related and comfort costs money.
The problem is that comfort and efficiency need to be seperated when it comes to operating costs. I suspect after reading the OP last post that comfort is paramount and money is not the issue but this contrasts the original post where he wanted his bills lower. That said, a good high efficiency system with comfort features, properly installed is what he needs, but by throwing promised savings into the mix it comes down to the contractor not addressing the comfort issue with the same level as the cost savings. The OP handicaps the contractor before he even starts.