I live in near the coast (sandy soil, high water table) and in a high-density, single-family neighborhood (0.25 acre lots). Geothermal units' performance here starts declining as the summer progresses, because the ground is getting hotter.

New high-SEER air-to-air systems are getting close to geothermal, with SEER rating of 20.

http://literature.broanhvac.net/Broan/HVAC/Air%20Conditioner%20-%20Split/FSA1BG/Technical%20Specifications/20_SEER_iQ_Drive_Ultra_High_Efficiency_Air_Conditioners_776e.pdf


As you reduce the cooling load through efficient construction, the pay-off from geothermal takes longer. That's why it's important to look at a mix of solutions, not just focus on one. Skim off only what works best from each technology... the Law Of Diminishing Returns. Similarly, going from 20 MPG to 30 MPG saves more fuel than going from 30 MPG to 40 MPG. (That's why our last car was a conventional 4-cylinder instead of a hybrid).