Well, I have gotten good results with 600mg with relativly fresh receptors, but after being on for a long time you stop making gains at the same doses you were using befor. When i made the jump to 900mg from 600mg I had been on for about a year and it helped bump the gains back up for me again.
You will absolutly see a dramatic dfference with higher doses, but you may not need to go that much higher if your receptors are fresh.