That hash rate would require their optional water cooling, I believe. That adds significantly to the cost and complexity.

Done some measurements as requested.
But again didn't dare the 1.3V. (maybe tomorrow)
But i am pretty sure that with 1.3V the 450Mhz could be doable.
The stabilizing effect of voltage adjustment is obvious with this dataset.
Core Voltage can be adjusted in fine steps just like your GPU, not just 1.1/1.2/1.3 like planned before.
The Water cooler did not have the impact i had hoped for.
But the undervolting tests were encouraging.
See for yourself:

ambient: 25°C, water temperature 26°C
Chip Frequency - Chip surface temp, cgminer board-temp, Core voltage, power consumption, comments

Air cooling:
282 - 45.8, 40.5, 1.2, 46W, stable
340 - 50, 43, 1.2, 56W, stable
372 - 51.5, 45, 1.2, 61W, stable
390 - 52.5, 46, 1.2, 2% hw errors
390 - 53.3, 47.5, 1.25, 70W, stable
409 - 55, 48.5, 1.25, 2% errors
409 - 56.5, 51, 1.28, 77W, stable
434 - 57, 52, 1.28, 82W, 10% hw errors

Water cooling by Anfi-Tec:
282 - 40, 30, 1.016V, 33W, stable 170Mhash/watt undervolting
282 - 42, 30, 1.085V, 37W, stable
282 - 45, 30, 1.20V, 46W, stable stock 122Mhash/Watt
340 - 48, 30, 1.20V, 56W, stable
372 - 50, 30, 1.20V, 61W, stable
390 - 53, 30, 1.25V, 70W, stable
409 - 54, 30, 1.28V, 77W, stable
434 - 57, 31, 1.28V, 82W, 4% hw errors

Project status:
First batch of boards is in production so I can react immediately when the chips are shipped.
Website - almost ready to launch.