Modelling personal vehicles would be really cool but would never happen as I would feel guilty at the amount of time and expense it would take PD to model my car and how much I'd be actually willing to give, which isn't a lot. Quite silly really.
That said, it would be awesome if you could model the car and also have real life upgrades.
I detuned the Lexus IS-F; painted the car a pearl paint closest to the Arctic Blue on mine, added weight, softened the suspension, and adjusted the gear ratios so 6 gears would only ever be used.
I don't think I'd pay to have my car in it as the above is enough for me. Also, I just need to step outside and into my car for the real experience, so in my opinion it's pretty pointless. Especially since my car isn't exactly what you call quick and probably wouldn't be fun to drive in GT5 at all.