If you like the 5K RiMac (hell I know I do), and can truly afford it, you should get one, they're amazing computers.
That said, I think that before you do you need to do two things.
1. Ask yourself why you have a Titan X in the first place
2. Evaluate your options
3. if you decide to get one, properly calibrate your expectations for gaming performance.
To start with #1, why did you buy the Titan X in the first place? Did you "need" faster FPS, want to game at 4K, or just had the cash lying around and felt like picking one up? If its the later, and particularly if its more than you need, the iMac may be the right computer for you. If its either of the former, you need to take a long hard look at what you really want and need in a computer.
Moving on, you already have a 4K monitor, a Titan X GPU, and what I assume is a high end Intel CPU. Have you thought about partitioning your boot drive and setting up a hackintosh partition to test the waters? While I imagine you're still planning on spending a lot of time in Windows 10 gaming, you should still see how you feel about OS X as an OS for everything else. Honestly if you're not going to use OS X, I really don't know if this is the right choice for you.
Some of your other options include
A. Waiting for the next iMac upgrade which should bring official support for Thunderbolt 3 and external GPUs (not to mention significantly improved internal dGPUs)
B. Buying a lower end 5K RiMac, selling most of your current system, but keeping the Titan X and 4K display to utilize with a thunderbolt PCIe enclosure, thus giving you both the gaming performance you're used to AND a 5K iMac (bear in mind games that are heavy on the PCIe bus will be slower)
Now, if you decide that, yes you want an iMac, you want it now, and none of the options I've listed above sound attractive, you should at the very least calibrate your expectations for gaming performance properly. As others have pointed out, there are resources all over this forum and the web evaluating the performance capabilities of the 5K RiMacs. Keep in mind, the highest resolution you'll be gaming at for demanding games (assuming you want over 30FPS) is going to be 1440p, and depending on the game, you may need to make some sacrifices to get the FPS you want even at that res. If you're used to gaming at 4K, just be aware, that aside from something like SC2 or Telltale's The Walking Dead, it's just not happening.
