Should've clarified that. When I used a "classic" nylon-walled inner and polyester fly (Eureka Timberline), I actually ran a test on night, hanging one thermometer inside and one outside. The temperature was consistently 10 degrees higher inside.

The last few years, my inner tent is 100% mesh (MSR Hubba or Carbon Reflex 1); the last test I ran only yielded a 5 degree difference.