I have never cast an 11' XP, but I have a 10' 6wt XP and 9' 7wt XP. My guess would be that you won't need any "ballast" to "balance out" an 11' rod. If you think of the length of the fulcrum from the grip to the reel, it would take a big weight change to offset a very little difference in weight of the rod (9, 10 or 11 feet).

If you have the opportunity to test cast the rod, I would start with your own reel and see how it feels, and with a larger one if it doesn't feel right. For me, the Ross Rythem 3.5 reel or Ross CLA 3 I use on my XPs both feel fine on either the 9' or 10' rods. These are very sweet rods, and I am guessing will not feel like you have to do anything to compensate for the rod length. Good luck, that should be a fun rod.