Iceboats are almost always sailed close hauled, and with the sail trimmed fairly flat. This is because they quickly outstrip the wind speed, and so, as you point out, the apparant wind shifts toward the bow.
It is for this reason, too, that you jibe downwind with an iceboat... running will not work. If you attempt to run, you reach the speed of the wind, and then your sail acts as a brake and "backwinds". So you jibe, close hauled on a broad reach, to move downwind.
As for the speed limitation, I do not believe there are any. Iceboats are the fastest, non-powered machines on the earth. I believe there have been some which have been clocked around 150 MPH, but that was a few years ago, and this may have been topped by now. Which brings me to the trivia question, "Who was the first President of the U.S. to have travelled over 100 MPH?". That would be FDR, who raced iceboats in his youth on the Hudson River, NY.
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