r300g is already faster than r300c, so I don't think that this is an architectural issue.
The problem with r600g is that it is really really new and immature, and it is also missing a decent shader compiler. I expect it to catch up with, and overtake r600c eventually.
If you are talking about the proprietary Catalyst drivers (which have nothing to do with Mesa), it's a different story, and the proprietary drivers are likely to stay faster for a long time, probably forever. There are many reasons for this.
On the other hand, the performance of the open source drivers (most likely the Gallium-based ones) will approach the speed of the proprietary drivers, but this will take some time.