4 or 5 ton systems should be multi-stage...better yet: Variable Speed!
Despite the passing of Regional Efficiency Standards, International Code requirements for Load Calculations & Equipment Selection, tighter homes & duct systems, I cannot see the likelihood of grossly over-sized equipment going down. The significant problem at hand is that HVAC Contractors are either going to use the system design process...or not.
In the meantime, Regional Standards have pushed Manufacturers into finding ways to make their systems more efficient. Other than variable refrigerant flow (VRF), I am not sure what else a manufacturer can do to increase the Energy Efficiency Ratio (EER).
This is all putting the burden on the contractor to find the room for installation, as well as finding systems that can deliver the latent capacity needed in the replacement market.
So, I propose this: Why don't we require any 4 or 5 ton system be multiple stage or variable capacity? This can address the significant over-sizing issue so often seen in the HVAC Industry. After all, a study completed in 2006 by the Florida Solar Energy Center: "Measured Impacts of Proper Air Conditioning Sizing..." found little electrical savings for homeowners when replacing systems 47 - 65% oversized. There is still significant peak energy savings to Electric Providers, hence the enforcement of proper sizing with most Utility Rebate Programs. I wonder what those savings to the homeowner would have been if they just installed a multi or variable capacity system? This code enhancement could change the industry significantly in future years, keeping the focus away from bigger coils and higher efficiency, but still pushing the contractor to proper sizing and equipment commissioning. Yes, multi-stage equipment costs more! So you should size the equipment correctly to avoid these extra costs, increasing your bids. Trust me, I have inspected as much as hundreds of A/C installations a year, for years, for a local utility rebate program. Almost never did I see an installation or replacement system larger than a 3-ton. Less than 10% of my inspected systems were larger, and almost always multi or variable capacity. Customers still reported increased comfort & reduced energy use.
Let's push efficiency using correctly sized, multiple capacity for the 4 or 5 ton systems, not physically larger units!