I think I personally would feel more receptive to the idea (in particular, Sulps' for VIP only) if house cars were more unique (clarified below); there's not much difference between a house car and a car you can spawn with /v. And based on that, why not just use /v?
The older members of SFSE may remember that we did have the ability to save tuning options for house cars, but it was
removed due to a critical server-crashing bug. If that could be fixed and people could save their vehicle tuning preferences, then by all means, let's expand the house car slots. Lots of people love personal vehicles / PVs, after all. I know it was a SEF feature which worked well.