Take another look at the Williams' site. Other than the vague statement "...there's performance to be gained," there's no information concerning inertia or equivalent weight. Of course, a driveshaft made of toasted marshmallow will weigh less and accelerate quicker than one of steel. But, what difference will that marshmallow driveshaft make on your timeslip?
I don't know if it was in this forum or another, but I posted...somewhere...the equivalent weight for a steel driveshaft with a size approaching that of "sewer pipe" and, even with those ridiculous dimensions, it was evident that, for someone in a category other than Formula One constructor, there are better ways to spend your money.