Being just 5 hours north of you, here in Dallas, with a 'down deep' earth temp of 68 degrees that I don't think is more than a degree different than yours, and with a closed loop designed at 300' per ton just like yours, here's what I'm currently seeing:

EWT ranges from 72 to 74 degrees, depending on time of day and load.  Or said another way, EWT ranges from 4 to 6 degrees above 'down deep' 68 degrees (you can see this at http://welserver.com/WEL0043/InstantaneousEnteringWaterTempsEWT-with2hrTCfilteronEWTs2.gif )

I would think you'd get similar results with a 300'/ton closed-loop design, assuming adequate diamter HDPE pipe and good installation.

And in another thread, I illustrated that we've experienced about a one-third reduction in our KWH consumption as a direct result of geothermal usage, all else being equal.  I would think it's reasonable for you to expect a similar reduction noting we have similar climates.

Good luck!

Best regards,

Bill