I really don't know about exact numbers, but of the 30 courses I think there are three distinct classes (albeit this is 3 classes among some of the best courses).
Top Class: Prestwick, Sandwich & Ballybunion
Lower Class: Machrihanish, Cruden Bay, Muirfield, Turnberry, Carnoustie and Lytham. None of these are great courses.
Middle Class: All the others mentioned. These are all still great courses.
I would replace the six lower class courses with any of the following: Woking, Enniscrone, Sligo, Royal Aberdeen, Formby, Addington, The Island and Portstewart.
Ciao