so while i don't have experiences in storing wind energy at sea or building islands. something that stands out to me as the same solution but i would think would be much cheaper and quicker. Do it the same way the oil platforms do it.
http://en.wikipedia.org/wiki/Spar_(platform)
the nice large vertical cylinder is used to store the oil until a tanker comes and then it pumps into the tanker, then it moves to a new well and refills the tank.
Using the same basic design, you would not need nearly the same build requirements as an oil platform because a potential leak just reduces the effectiveness and isn't an environmental issue. Also using this design they could store them closer to the wind farms, move the potential energy as needed, and you could make them unmanned units which would further lower the cost to build.
for shallow waters the oil companies use the same design but rather build a concrete cylinder that is connected to bedrock, this is a permanent storage platform that normally gets it's oil from multiple small wells via pipelines and gives tankers a central point to load up. they could possibly use that design in place of a sand built island.
Also note, that you can just go out and buy a platform, new or used, (lot of old used Russian ones on the market). which could accelerate the time to market, and reduce their engineering overhead.
again, this seems a simpler solution then trying to build a sand island.