Well, I did some tests myself 2-3 days ago, while having to make the pass from Bergen to Scapa in 1945 - Man! So many planes even after the first version of the NYGM Airmod -. The XXI after the fix goes well above the 340 @5 kn. (as per Uboat.net). The battery reaches 10% and you continue @ 2-3 kn. for about 150 miles more before the battery is depleted.
Even after the message "battery empty" the uboat continues to go forward @ 1,5 kn, but then you must surface due to CO2 levels.
Apart from the XXI which battery is definately flawed, I have the gut feeling that all u-boat batteries are mal-modelled (esp. from 40% to 0%)
I will do a test during the weekend and might make a global fix for all sub types; but this needs some time.
Any other report from testing Kaleuns would be valuable. 
If this proves true (the flawed modelling), it's going to be a real realism hit; The depletion should be a little more aburpt. A lot of u-boats in RL had to surface after a long hunt by DDs.