Except for unibody vehicles like my Defender which can tow over 8,000 lbs but is not supposed to ever use a wdh per guidance from Land Rover.AFAIK, 5,000 lbs. is the limit without a WDH.
IOW, a WDH is required for more than 5,000 lbs "conventional" towing regardless of the total tow rating (even HD pickups that are rated to tow upwards of 18k lbs.)
So I wouldn't assume that having a WDH will increase the tow rating.