Well the userbase argument doesn't work for openers. Obviously, smaller userbases with popular brands just means a more concentrated base for the game which leads to bigger openers. Such as the case for Halo 3 back on the 360 which had amazing attach rates. Or even Zelda: Twilight Princess launching on Wii with above 90% attach rate. GT5 will probably have a relatively big opener given that.
However, userbase does matter when we deal with "legs". Obviously the GT games are huge mainstream titles. And the longevity of the sales of the title could be affected by a smaller userbase. But we also need to factor in the potential of the game becoming an "evergreen" title or system mover. That could extend the life of the title.
It's hard to tell now if it will sell the "worst" in the mainline series, but I think given the past history of PS3 buyer trends, it should open quite well. I don't think it'll sell bad given the PS3's situation and possibly be able to go over that 10 million mark. Just going to need more info on PS3's state 6 months from now and a year from now which obviously I can't know until it happens haha.
