There were ~2 years between GT Concept and GT4 Prologue, and the latter sold 2.24 times as much as the former (748k vs 334k). If GT5 were to exhibit the same rate of increase, it could come close to GT4.
OTOH GT4 Prologue to GT4 final only saw a unit increase of 43% (1066k vs 748k). However, there was only one year between these two releases. It's already two years since GT5 Prologue Spec III.
Speaking of which, GT5P S3 is only 23% of the original GT5P (295k vs 239k), with a year's gap between the two releases. That'd be a hint on the other side, speaking against strong GT5 sales expectations.
Point being, releases in quick succession "graze" away a lot of the demand. With a two-year window since the last stopgap release though, the final game could see a healthy increase.
garaph link
Keep in mind that PS3 install base more than doubled since late 2008.