I can imagine that the genetic drift evolution process could take a very long time, especially if the plant or animal were already at maximum or near maximum evolutionary equilibrium!
That is of course a good point and something I hadn't considered when I had asked my question - the lapse of time between generations. Of course organisms with a faster breeding rate and more offspring are going to have the ability to evolve in a shorter time frame.
But if we simply take the number of generations into account, leaving time out, I would expect that no organism could potentially evolve faster than other. But it depends on so many factors that I don't think it's really possible to demonstrate that!
I'm reading one of Dawkins' books at the moment, it's covering all of this and more and makes for fascinating reading. Plus I've got several more linedd up waiting to be read.