Wow! It sounds like you've got a real winner there!!
If you cannot be bothered to change EPs, and just looking for a comfy cruise of the night sky, the 17mm T4 is certainly the EP for the job in the 12" Dob.
I am surprised to read that you see more detail in galaxies like NGC 253 with the 17mm T4 than with your shorter FL Pentax XWs. After studying an object for a while in the 17mm, I suggest you swap over to the XW-10 for a closer look. In my experience in side-by-side comparisons in the 12" f/5 Dob I could see more detail in galaxies with the XW-14 than with the 17mm T4 and more still with the XW-10, and sometimes a bit more again (or more precisely, the same detail seen easier) with the XF-8.5. The XF-8.5 rules on most planetaries - well not big ones like the Helix obviously
. (My XW-7 is a more recent acquisition and I have not used it enough on faint fuzzies to tell where it fits in.)