Found something in the British Admiralty Intelligence Division Report on Interrogation of U-Boat Survivors, dated June 1944.
Section One details information on all U-boat specs, page 12 is for the IXD2.
The reports says 25000m@10kts with 475t of fuel, as opposed to 10000/15000m@10kts with 139/216t of fuel for the IXC variants.
There seems to be a lot of other good stuff in that report too.
http://www.uboatarchive.net/CumulativeEdition.htm