How do you benchmark an Arduino?
No.

That is because millis will be equally affected by any frequency change (due to temperature) as the benchmark code. You would need an external clock source that is precise enough.

Note that temperature doesn't have that big an impact on crystal oscillators. Somewhere around 10ppm between -20 and +20 degrees C. The crystal itself is probably already 50ppm precision to start with (higher precision ones are more expensive).