Ceilings -and walls, for that matter- are rated as complete assemblies, with materials and construction details specified.
With that in mind, it's quite possible that a particular fixture might be fine for one ceiling, but not another. Most assemblies use 'ordinary' fixtures.
The same applies to construction details; one ceiling might require the fixtures to be 'boxed in' with drywall, or covered by insulation, while another will not.
Is there any value to 'improving' things with, say, better fixtures? Probably not. Since the entire ceiling is tested as a complete assembly (I think the test section is something like 15ft x 20ft), a change to one small section probably won't affect the test results much.
