Donnan's UGAG record prior to Richt was...
1996: 5-6/3-5 (taking over for Goof, win over GT)
1997: 10-2/6-2 (win over GT)
1998: 9-3/6-2 (loss to GT, otherwise 10-2)
1999: 8-4/5-3 (loss to GT, otherwise 9-3)
2000: 8-4/5-3 (loss to GT, otherwise 9-3)
Richt's UGAG record...
2001: 8-4/5-3
2002: 13-1/7-1 (SEC Champ)
2003: 11-3/6-2
2004: 10-2/6-2
2005: 10-3/6-2 (SEC Champ)
2006: 9-4/4-4
2007: 11-2/6-2
2008: 10-3/6-2 (loss to GT)
2009: 3-2/2-1
Tossing out his first year, CJD was good for 8-9 wins (with a loss to GT) and 2-3 losses in conference. CJD was 2-3 against GT, 1-4 against UT, and 1-4 against Spurrier's UF.
Tossing out his first year and the high-low outliers, CMR is good for 10-11 wins (with an extra game, and a win over GT) and 2 losses in conference. CMR is 7-1 against GT, 5-3 against UT, and 2-6 against UF.
The difference between the two is not huge. Richt is , on average, 2 total wins and 1 SEC wins per year better.
CJD happened to coach against George O'Leary at GT, and Richt had the luxury of coaching against CCG. CJD coached against Spurrier's UF and Fulmer's UT teams at their peaks. Both won MNC's during his tenure and both were in his division. Additionally, GT was at its 50-year peak, too, winning an ACC championship during that period as well.
Clearly, CMR has made his difference against an extra cuppie due to the 12th game, a medicore GT under CCG, and a sinking UT program. He hasn't won a division since Urban Meyer's first year at UF. He won two SEC CG's prior to that time, and played in a third.
I don't think you can say yet that he is really any better coach than Donnan. He has had the good fortune of all three of his major rivals going through major transition during his tenure, as well as having a 12th game added to his schedule. Put him in Donnan's shoes and I'm not sure he fares any better.