One problem of building so many in one area (within a couple hundred kilometers?) is that there is less geographic averaging of wind. When the wind dies there, likely there is very little production.
That probably limits it to some percentage. Would be interesting to see some summary what kind of other measures are required in the grid, the bigger it gets. Things like flexible demand, pumped storage (all the countries are flat except Norway).
This is partially mitigated by double use of the island's connectors as interconnects. If energy production on the island dropped to zero, the entire cable capacity would be able to move power between connected nations. The 'flat' United Kingdom has 2.8GW of existing pumped storage capacity. (http://www.british-hydro.org/hydro_in_the_uk)
If this was the only power generation in the area it would be an issue, but even such a large power plant as what they propose would be a highly effective addition to the grid of the north sea area.
That probably limits it to some percentage. Would be interesting to see some summary what kind of other measures are required in the grid, the bigger it gets. Things like flexible demand, pumped storage (all the countries are flat except Norway).