More of a question than an answer, but should spoke tension be checked as a possible factor? Compare tension in the flexy wheel to tension in the other wheel (Williams) that the OP has?
(I believe I could output 1000 w for about 1/1000th of a second, so no answers based on personal experience here.)
I had my mechanic/wheelbuilder check and adjust the spoke tension of both wheels. I forget what he said the number was, but he was happy with both.