Had my first ride today on my new carbon bars, and thought I'd report back on the experience for any others thinking of getting them for numbness reduction.
Picked one of my longer, faster, hand numbing trails to test them on - a 43km rail trail with mixture of hardpack and old railway ballast which is always guaranteed to bring on hand numbness within about 20 mins.
The new carbon bars didn't feel any different at all to the old ally ones, so I was beginning to wonder if I'd wasted my money 
but then as the ride wore on into it's second hour I realised I hadn't experienced any numbness - result 
For the doubters - everything else was the same - tyre pressures, suspension settings, grips, sweep/angle/rise of bars, although the new ones are 710mm versus 690mm. I didn't feel any extra flex, but they must have been absorbing the worst of the vibrations for my hands not to go numb.
So, I'm chuffed, and wish I'd made the switch years ago