Its pretty obvious looking at this graph that the Mercedes engines, in all cars, were not running at their highest potential power outputs. The possible exception being the Williams.
The biggest giveaway is the shape of the Williams distribution. Its bi-modal, indicating that Williams were likely running two power modes.
All of the other Mercedes-powered teams have a uni-modal distribution, which would suggest a single power mode was used throughout.
As to the speed hierarchy, excluding the 2nd mode from Williams, the speed ranking would be:
McLaren
Mercedes
Aston Martin
Williams
This wouldn't be entirely inconsistent with prior data.
Furthermore, if the bi-modal data of the Williams is representative of usage of a higher engine mode, then the average differential in top speed between modes is +-15km/hr.
Incorporating this into the observed data, we could see average and peak top speeds as follows:
McLaren: 320 average, 328 peak
Mercedes: 318 average, 325 peak
Aston: 317 average, 325 peak
Speculation on my part, but lets see what transpires. I don't think any of the teams were running 'full beans', more likely 'peak' sustainable race power (e.g. Ferrari and Red Bull), so perhaps even higher values may be achieved.