Djokovic is also worse at RG than Australia, so that doesn't mean a whole lot.
And, I don't think there's a huge difference between Federer on slower hard courts and clay. He accomplished more at Australia primarily because he never had to face anything like 'Nadal on clay' there. If Nadal had not been in his way, he could have easily exceeded his Australian Open accomplishments at Roland Garros.
Look at Federer's results in his prime stretch (2004-09):
Australia - 2004 (W), 2005 (SF - Safin), 2006 (W), 2007 (W), 2008 (SF - Djokovic), 2009 (F - Nadal)
Roland Garros - 2004 (R32 - Kuerten), 2005 (SF - Nadal), 2006-08 (F - Nadal), 2009 (W)
Wimbledon - 2004-07 (W), 2008 (F - Nadal), 2009 (W)
US Open - 2004-08 (W), 2009 (F - del Potro)
His only non-Nadal loss at Roland Garros came against the second best clay court player of the last 25 years and one of the 5-10 best of the Open Era. I'm not arguing that he's better on clay, just that the gap isn't that great. Minus Nadal, it's relatively safe to assume Federer would have at least equaled his Australian Open achievements at the French Open.