Turkeybaster, Lap times are useless? Really? I'm not trying to be an ass but how do you sugest proving that you're stressing the car? You can always find clear laps during a track day.
Your car will be close to maximum temp when you start a clean lap. Our cars heat up so fast 2-3 minutes of clear track in the middle or end of a session is all you need.
I suggest measuring your time against the winning spec Miata times. Of course not all tracks make sense for this but you need something as concrete as possible and Miatas are a good comparison.
Use Trackpedia to find them.
For instance:
Track } Me } Spec Miata
Infineon } 1:58.6 } 1:56.3
Laguna } 1:47.6 } 1:46.6
Thunderhill } 2:09.7 } 2:08.2
One other thing is that Cobb, GIAC, Procede, et al give you lower lap times without driving harder but can also heat up the car faster so it could be pertinent to list tunes if they exist. It also will help those with tunes understand what the best solution for them might be.