No, they're ports of the original models from GT1. It's partly automated, but there's a decent amount of manual work needed on top to add a GT2-style shadow and scale it properly, patch in some of the semi-unknown stuff to do with LOD distances that GT1 doesn't have, and correct the wheel positions. The textures also need GT2-style wheels to be added - the NSX uses a set from the wheel shop edited to centre-lock, and the Silvia's are from some racecar somewhere, I forget exactly which. Physics-wise I tend to use a similar donor car from GT2 as a base, then port over whatever I can from GT1 such as the engine, transmission, aero balance, and so on.
Yeah, these two cars could have been serious show-stoppers if I couldn't get the models fully functional, and it has taken a decent amount of work today alone to shake out the last few major issues. The current models are something like the 4th or 5th time I've ported them now, improving the process each time to gradually get them fully functional in GT2. They're not necessarily the last hurdles though, I have had releases stopped at the 11th hour in the past because of totally unforeseen game limits, so I don't take anything for granted until the final build of the release is running on a PS1 console.