I went for the 13" Bitubo's when I installed my first pair back in 08 or 09. There is still a misconception that installing the 13" over the 12" will raise your bike 1". That is false because of the shock angle, my seat height increased slightly more than 1/2". Well worth the height difference for the extra clearance and additional suspension travel. I would definitely try swapping them out for a definitive answer as to what difference they will make for your specific needs. Heck it's like a free test ride before you drop the big bucks for a high quality set of rear shocks. I have ridden allot of bikes and I honestly believe that with both bikes set up to the best suspension upgrades you can make the Road King will be the choice for any type of high speed riding versus the Ultra which will be the best over rough roads and longer rides. The Ultra will give you a better ride over long distances due to it's ergonomics and wind protection. That is not to say that there are some Ultras that can't keep up, I just believe the Road King lends itself to a more spirited ride. You have the best of both worlds, and modified for the kind of performance anyone would like to have.
If you do the swap you may wish to try them as they are before you adjust your pre-load, compression and dampening for the lighter RK. There is a sweet spot, you will just have to make a few minor adjustments to find it.