Anyway IXD2 subs fought during World War II achieving some success. Fourteen o them sunk 96 ships for a total of 522523 GRT in two years (1942-44). So they should be also capable to do that in NDT.
As for diving times Now I have found on U-boat Archive such data:
1. Type XB: Best diving time 37 seconds to 20 meters, average diving time to 20 meters 40 – 45 seconds
http://www.uboatarchive.net/U-234INT.htm
2. Type VIIC: Before platform I was widened and platform II added, i.e. prior to last patrol, U-664 dived to periscope depth, 13 m., in 18 seconds. After changing the superstructure the diving time was increased to 28 seconds.
http://www.uboatarchive.net/U-664INT.htm
3. Type IXC: Prisoners said that the best crash-diving time obtained during trials in the Baltic was 38 seconds to periscope depth.
http://www.uboatarchive.net/U-187INT.htm
4. Type IXC/40: At Hela U-841 crash dived in 35 seconds. On patrol she could not do better than 41 seconds due to increased resistance of larger gun platform and of splinter shield on quadruple gun mount.
http://www.uboatarchive.net/U-841INT.htm
5. Type IXD2: 38 seconds to periscope depth at flank speed.
http://www.uboatarchive.net/U-177INT.htm
So I cannot notice IXD2 had significantly longer crush-diving times than another Type IX variants and large Type XB mine-layers. It was most probably about 40 seconds on average. Apparently the rule of thumb is Type VII U-boats could dive two times faster than Type IX subs.