Only 2 of those were modeled for GT3: The 911 and the Nomad. Dunno what video you saw, but it was a fake. The other secret car in that game was the Stratos.
Your theory is entirely possible though; considering how quickly Ferrari was announced after GT4, it's entirely possible PD's created a few Standard-level models that we've not found. Though they've never shown up hidden in games like the GT3 cars, since GT4 and PSP haven't shown anything...
Oh jeez, I forgot about the poor, awful Silvia. A perfect example of how important a good set of rims are 👍
Also... very cool to see one of the "Specials" back and in a proper racing pack. I have found at least one Standard I'll be enjoying

. It looks like PD still uses power-to-weight ratio as the ultimate apparent equalizer, which just doesn't work with such a light car, even if the theory is sound.