I acquired an X300U yesterday, and took it to the range last night. This light is pretty amazing and you're going to want one.
I brought one of my standard X300's along to compare output to. The standard X300 is one of the newer 170 lumen lights. As many of you know, my math skills are practically non-existent, which of course extends to reading a tape measure. One of the major concerns about the new Ultra is whether or not the extra length was going to be an issue. The X300 from bezel to end of body, minus the tail cap, is just about three inches. The X300U has a crenelated bezel that the X300 doesn't, so I chose to measure it from the peak of a crenelation to get the max length. The same measurement for the Ultra is about 3 3/16 inches.
I have a few holsters that fit pistols with lights, several by Safariland and an older Blade-Tech for an X200, and currently all for Glocks. A couple for the G35, a few for the G17. All fit without issue. The 6354DO fits it especially well, with the mechanical spring ALS unit that holster uses.
The difference in output, 170 lumens for the X300 vs. 500 lumens for the X300 Ultra, is dramatic. It's very difficult to quantify with words, so I put together a short video of a couple of horrible plate rack runs that shows the difference much more clearly.
One thing I continue to find out: fiber optic front sights are useless in the dark; they just disappear. My 45 year old original equipment eyeballs can't see the dang thing. If you're gonna fight with a gun at night (and if you know you never will, share that trick with me), don't use FO front sights. Spend the extra duccats and get a tritium front.
Here's the video I mentioned earlier. I did, in fact, choose the most irritating music that Flip had available: