Right, I just spent some time seeing which Standard cars can easily be converted to Premium. By that I mean the car already has a Premium counterpart (e.g. the Standard M5 and Premium M5). Little tweaks and different liveries may be needed for its looks, but for the most part, the car is already modelled.
You forgot a few, actually
AC Cars: [1]
Acura: [1]
RSX Type-S '04
Alfa Romeo : [2]
147 2.0 Twin Spark '02
147 GTA '02
HKS : [1]
Genki Hyper Silvia RS2 (D1GP) '04
Honda : [3]
Civic Gathers Drider Race Car '98
Integra Type R (DC5) '03
Integra Type R Touring Car
HPA : [1]
Golf R32 Stage II (SEMA Award 2004)
Subaru : [5]
Impreza Cusco Advan JGTC '03
Impreza WRX STi (Type-II) '02
Impreza WRX STi Prodrive Style (Type-I) '01
Impreza WRX STi Spec-C (Type-II) '04
Impreza WRX STi Version (Type-I) '00
Total Cars: 517
I could've included basically every version of the Lancer Evo too; I through IV shared many parts, V and VI are very similar, and VII through IX all are essentially facelifts as well. I included the Scoobies on the basis of you including the R33 GT-R Road Going Version from Nismo: that would take a far amount of remodelling, at least as much as punching out some new noses and taillights for the existing '05 STI
. On that basis, the 147's may be a bit too different from the '06 model, but I can't imagine so. The Integras/RSX is a simple facelift.
I'd say the front-drive Clios/Lutecias probably shouldn't be on the list: they're very different cars from the mid-engine lunatic. But either way, your list is awesome, for pointing out how easy it'd be to convert a vast chunk of GT5's car list to Premium: 517 (and more) just using the current Premium lineup as a base is impressive. Even more when you consider, in reality, we don't need every tiny permutation of a model, but even if PD wants to keep them in, a lot of their work is pretty much done for them.
It'd be interesting to compare your list with the "erase the duplicates/very very similars" list that floats around, to see how many actual unique cars PD would need to work on.