You won't gain a huge amount I can tell you that much.
Dry soil has a natural specific heat of ~ .2 and wet soil is around .35
http://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html
Also the relation of heat pump system operation Vs soil conditions has been extensively studied and documented as well.
http://www.google.com/search?q=soil...=chrome&ie=UTF-8#q=thermal+properties+of+soil
http://www.eng.usf.edu/~gmullins/downloads/Thermal/Thermal Properties of Soils.pdf
Realistically being under your sewer drain field runs will help with the thermal mass and some thermal conductivity (but not a huge gain on either) but if you are using the system for winter time heating you could run the very real possibility of freezing your sewer drain field solid rendering it unuseable in the dead of winter.
I haven't worked with geothermal much myself but I have talked to enough people who have dealt with it and they overwhelming view is that you take whatever your contractor says is sufficient for your ground loop and at least double it.
Reason being very few places ever have exactly dead equal summer/winter heat sink/source ratios let alone a natural ground temperature that's favorable to both so at some point after a few seasons of regular use you will end up with either a thermal saturation or thermal deficiency issue in your effective ground loop assuming nature itself has not already put a heavy bias on one end to begin with causing your system to stall at one or the other early.
Where I live our natural ground temperature is ~40 degrees which makes geothermal here useless for anything but summer time air conditioning no matter how big of ground loop is used.
Many here have tried and many contractors got rich and keep getting richer from those attempts but so far no one who has had a system for any length of tine says it was as good as the sales and contractors said it would be for heating.
http://www.greencastonline.com/tools/soiltempmaps.aspx