From the Gamma Sigma Alpha (which sounds sort of like Order of Omega) site at USC. Data is from 2002:
http://www.gammasigmaalpha.org/spring_2002_results.htm
Clarkson University
3.7% above the Non-Greek GPA
(2.9905 vs. 2.8831)
Brenau University
5.6 % above the Non-Greek GPA
(3.23 vs. 3.06)
Mt. Union College
6% above the Non-Greek GPA
(3.233 vs. 3.049)
Loyola Marymount University
2.61% above the Non-Greek GPA
(3.15 vs. 3.07)
Those are just the first 4 listed. There are about 25-30 more schools listed on that page, and those are only schools with Gamma Sigma Alpha chapters.