The thing is, I didn't say that it should be prioritized. And even in my original post I said that they already did it with the Miatas and the Aston.
I am not an expert in 3D modelling, but how much time could it take to take the premium BMW M5 model and make different side mirrors (because that seems to be the only exterior difference between the '08 and the standard one)? Or the '69 Camaro SS, another ugly standard model that could be a premium by changing some badging or trim on the premium Z/28? PD's modellers have always been among the best, I doubt making changes that small would take a modeller more than a day or two, if not only a few hours.
BTW, In my opinion, the whole semi-premium thing is unfortunately a waste of time. Most of them only have slightly cleaned up textures or a bit of added geometry but it's still nowhere close to premium quality. Even the best ones, that seem to actually have been remodelled like the Yellowbird need work. A huge majority of the so called semi premiums only have slightly cleaned up textures or a bit of geometry added. These improvements might barely work for GT6 but not for the next generation.
It's really quite a mess PD have made with all of the quality differences.