"Street legal car" should be considered a car that is/has been legal for street driving in the country it has been sold (or brought to use). Not all cars are legal in every country.
In my OP I talked about cars like:
Amuse S2000 GT1 Turbo - A Touge-racing purposed but street legal in Japan (was owned by Hideki Tanabe).
R390 GT1 Road Car - 2 were made and one has been driven legally on streets of Australia. Other on went straight to museum.
Tommy Kaira ZZII - Was a 90.000USD production car in Japan
Viper SRT10 ACR - Sold in US
Speed 12 - One was built as a street legal in UK. You can even find a video from Youtube where it is driven on the streets of London.
Cadillac Cien - One fully working, road going version has been built but might not be street legal anyway so you can cut this on out if you like...
W12 Nardo - Several Protos were built but probably not one street legal version.
So "production car" and "street legal" aren't the same thing. In my OP I was talking about the latter.
I think that RM or rally cars shouldn't be considered as street legal because they surely aren't. I still think that it's ok to do all other upgrades in GT Auto and they'll still be street legal cars. I know that slicks aren't allowed on roads but it's for the sake of better lap times in the game.
And yeah the Zonda R isn't street legal. I don't know why people keep thinking it as a street legal. There is the Zonda Cinque (not in this game) which is the street legal version of it.