No, it is not a valid test.
There are many variables that would impact performance in a real product. Even if you were making a game about lots of physics boxes it would be more important to know what framerate the desired gameplay ran at than if it declined at 5000 or 6000 boxes. Especially if the design never called for more than 1000 at a time.
Also, in the current year the PC crowd would not be happy with a game that only ran at 60 FPS when they own hardware that can play other games at 144 FPS. So designing a test around the assumption that 30 or 60 FPS is sufficient is also flawed.