Testing is such an interesting phenomena. First week no doubt you bring out your test mule, you swap out parts, put more on the car, compare them back to back, correlate your findings. The drivers are told to do cheeky things, not stress the power units, get familiar with the behavior of the car. Just so many different things have to get crammed into 8 hours plus a lunch. Not to mention the mechanics earn their damn checks during testing, getting familiar with the engine installation, you'd be surprised how well mechanics input can be to chassis development, and overall integration of the power unit. The men and women who build these cars know every crack and seam.
Anyway, I imagine the test mule has a lot of adjustable bits that would be far more efficient if more "fixed", lower weight, better performance etc. I wonder how that combined with the engine let loose will look like. I will be pleasantly surprised if we see 16's.