Assuming GT is released 2010, the PS3 userbase should be above 30mil. In fact, if released late that year it may even be approaching 40mil. 10mil wouldn't be impossible at that point. The userbase doesn't entirely determine how well games will sell.
If you don't believe me, just look at the N64. In the US, about 20mil N64's were sold, compared to over 40mil PS1's. Yet, look at how the top 5 selling software ended up looking like:
Over time Gran Turismo has developed into the Playstation's "top game" (unincluding GTA of course). The smaller userbase won't be quite as damaging as you seem to believe. We don't have complete data for GT3, so I'll just give Japan's numbers: when Japan had 5.6m consoles sold, GT3 sold about 1.7m. That's a 30%+ attach rate. Should be high enough, don't you think?
