This 40% rule is for heat pumps. It's not restricted to geo systems. The heat rise is lower with a heat pump than most other systems. More air at a lower heat rise has to be moved in order to have an equivalent heating capacity.
If the duct work is not sized properly you can have a system that is noisy and feels very uncomfortable even though the system can maintain the set point on the T-Stat. This is the most common complaint about heat pumps.
The problem is actually compounded in a retrofit because most duct systems are undersized to begin with. A natural gas unit with a supply temp of 120F. feels warm even at high velocity. A contractor/builder can add profit by undersizing the duct system with few complaints.
A heat pump with a 90F supply temp will feel cold if you are in the air stream. Your wife wont care if the building temp is "correct" if she feels cold.
The heat pump should be sized according to Manual "J".
The duct system should be sized according to Manual "D".
If your contractor is not using industry standards you will likely end up with a poorly functioning system.
