There is no one correct answer. Some real cars are better finished than others, and when in a timeline makes a difference too. Pre-race versus post-race for example. Plus often presentation and restored cars are detailed better than workhorse race cars. I have seen some race cars that look shabby, others that look fantastic, at least before the race.
Then you have the issue of when you scale something down, the surface becomes finer, maybe definable as shinier. And that is somewhat subjective too.
As far as “contest models”, what defines them from other models? I’m not sure why different approaches need to be taken if that was what you were getting at.
Build it the way you want it to look for yourself, and if you are building for someone else, find out what they want and try to match that. If you are building to please some arbitrary judges, you’re on your own; I got nothing for that…
While it is true that many (most?) of the older cars were not clear coated, it doesn’t mean that by using clear it is automatically not going to give you the desired results. Just as many real cars that are clear coated can be realistically replicated in scale using other paint methods, and no clear. Factor in decals with different sheens than paints, and other inconsistencies… To me, each project needs to be assessed on its own merits.