It's not hard to believe. The US147 is easily competitive against 82 and 85 wide slalom boards, especially in the 8-14 knot range. Many of the sailors in yesterdays PWA event in Turkey would have been more than happy to have been allowed to use one instead of struggling out of the gybes!
In 16-20 knots certainly 8.6 and 68-76 wide boards will be faster. Time to change the rules and allow such big slalom boards before PWA slalom disappears in venues where the wind could be 8-14knots