Pre-orders for the HTC U Ultra don’t even start shipping in the U.S. until March, but TWRP is already available for the device. However, it’s not an official TWRP version but a beta ported over by recognized XDA Developer @Captain_Throwback.
As such, there are no guarantees that the custom recovery will work exactly as intended, but it will allow you to get TWRP 3.0.3-1 on your U Ultra the minute you unbox it. As it stands, the only known issue is that the reboot recovery option is not yet functional, but this will be fixed once the dev has access to the kernel source.
As always, there’s a lot of reading material over on XDA if you’re interested, so be sure to read up before proceeding. As always, if you’re using your U Ultra for a while before taking the plunge, be sure to back up any essential data first.