Clarrie,
The ETOPS range rings are a planning issue calculated using a quite complicated process.
The upshot of the calculation is that the speed is really irrelevant as Chuckles says.
Wind is not considered and the range rings remain the same regardless of the weather conditions (ie enroute winds etc) on the day.
As it is purely a planning exercise if you end up on one donk you do whatever you need to do to achieve the desired result - you are not committed to operating at the speeds indicated.