Weren't that many? GT6's Premiums were similar number to what's probably GT7's car list will be like; GT6 had 447 Premium cars. And probably Enzo is a case of Ferrari being Ferrari.
447 looks like a big number, but once you start digging, you'll see that it's not really that many, specialy if we are counting the tunable ones. GT3 cars, GT500, GT300, LMP1, Group C, WRC Rally cars, Nascar, race cars and touring cars that don't fit in none of these, SEMA winners, Pebble Beach winners, VGTs, Senna cars and kart, Karts, tuner cars, Red Bull X cars. Then there are the base models of some of these cars, then the stealth models and Chrome Line cars. Then specific cars that can't receive mods. A quick check, shows that around 250 cars in GT6 can't be modified, the remaining that can, they don't have the same amount of mods, some are more limited than others. In GT7, assuming we are getting all the GTS cars ported over, it won't be much different, given the amount of race cars, VGTs and so on, that can't be modified. On the bright side, we don't have duplicates of the same car with different liveries like previous games.
That’s what I figured. Cars returning, are already modelled. How long does it take to add a race car interior to cars that have Gr.3/Gr.4/Gr.B variants? Plus, if there are nearly 100 new cars, how many are road, race, VGT, concept cars? All of those may not need to have racing parts. Maybe a split of 30 new race cars, 30 new road cars and 30 of VGT/concept. Adding 30 new interiors and race exteriors to plain road cars, is not a lot to do over three to four years.
Again, if PD are utilising out sourcing, they’ve only got the percentage of half the car list to do. The other half already being race cars.
Re-using the interior of a race car on a road car could be a solution, but it won't be available to every road cars. Though I wouldn't be surprised if roll cages were limited to cars that already have a race version, be it Gr.3, Gr.4 or Gr.B, taking advantage of the existing interior model.
Doubt we will get one third of the 100 new cars, as concept cars and VGTs. There aren't that many VGTs left, and PD are so behind on up-to-date cars, that they can't afford to waste resources on concept cars, except maybe a few exceptions.
Speaking of resources, the problem is not them only having "half" of the cars to model race interiors for, is that the modelers doing that, won't be modeling new cars, be it updaded premiums from GT6, converted standards or completely new cars to the franchise. The car list being fairly limited, might be enough for them to focus on new content.