Think you will find that the insurance costs associated with young drivers are those relating to third party damage and in particular injury. Hence the reason that the car the youth drives doesn't make too much difference to the premium unless of course it is a powerful car where it could cause...
As a driving instructor, I have to agree with this. The scheme had never really worked as insurance is too competitive and they undercut each other to buy in the business. You might get one company reducing their quote because you have the pass plus and still get a cheaper quote from another...