If the car isn't allowed in ST, it isn't allowed in STR or STU. Under your
scenario, ANY car would be legal for STU. I understand how you are
starting with a derivative of a legal car, but the fact remains that if the
car isn't eligible, it doesn't matter what family lines it has. You must
look at it like this: GSX - illegal for ST, Viper GTS - illegal for ST.
They both are still illegal for STU...
Wait a minute, are we assuming that you can add a turbo where there was
none before in STU?
But what if it has a normally aspirated sibling that IS allowed? If the
NA version of the Eclipse is allowed in ST, is its big brother (either
AWD or FWD) allowed in STU if the only differences between the 2
would be legal if you started with a non-turbo in the first place?