Thanks for your input. The idea of installing an undersized condensing unit doesn't make sense to me either. The high efficiency residential units I've seen have oversized condenser coils. So it seems better to use a 5 ton condenser coil with a 3-4 ton compressor to get better heat rejection.

A fixed output compressor is just that - fixed output. So the energy savings has to come from other components. Reduced fan loads and better heat transfer through both coils seems like the obvious (cheapest) choice from the designer's point of view.

Any mixing of components requires complete analysis of the process. That is not likely to happen at the installer level.