I think you read my other post in "GT5 will fail...." for that 40% better. GT4P sold 40% better then GT5P in Japan, and it was released only a year before the launch of GT4, where GT5P had 2 years.
As for user base, I don't really corrolate sales with user base. As Halo shows, Halo 3 had a 40% attach rate, but Reach only has about 12%, and it's still selling better. Early into a consoles life, there is little to no competition, and games have long periods of time to sell. Thats why I think GT1 and GT3 sold better then GT2 and GT4.
The only attachment rate arguement I make is that GT1 and GT3 sold better then GT2 and GT4, and GT5 is being released late into the consoles life, like GT2 and GT4. Also with PS3 total sales likely to be about 2/3rds PS1 sales, and half PS2 sales, it would be substantially more difficult for GT5 to sell as well as prior GT's. I mean GT's on PS2 did outsell GT's on PS1 generally, so I expect GT5 to sell 1-2 million worse then past GT games.
Yes, GT3 won't be reachable, most probably. As I wrote in one of the other threads, current 2nd and 3rd best selling GTs are very close, so GT5 getting 3rd place as final rank is very difficult: IMVHO, doing the launch perfectly and having very long legs, it could get 2nd place, otherwise, for example botching the launch, but with legs and a PS3 staying alive long enough, it could eventually get 4th place. Partially lacking more than one selling factor, it will get 5th place, but not lower, as the current 5th is GT5 Prologue and GT5 will sell far more than it even not faring well.