The effect of freezing on taste is certainly subjective, but it is not seen to be detrimental to the taste or texture. In fact, several renowned masters are in record as preferring there fish to be raw at purchase and then they freeze them themselves using low temperature deep freezers for set periods of time. I do find the effects of freezing to enhance the preparations, but then I tend to agree with the Japanese philosophy that sushi is about the rice, and the fish is just a condiment.