Wait? No.
There will be incremental improvements all the time and nobody is telling the public when or what, but the quality is there and performance already exceeds the range needs of anyone that doesn't have long distance towing as a deciding factor.
Faster touchscreen (MCU) and comfy ventilated seats are nice to have, but again, not essential, just nice to have, like DL, nobody needs Insane, let alone Ludicrous, but (very) nice to have. Don't wait.
It will be years before a "wow!" update to the X. Once the Roadster or Pickup unveil the next generation batteries, then we'll see that go into the S3XY cars. That's when I'll update.
Next is the Y, then the pickup, Semi, and the Roadster ... all "by 2020" ... who knows ... maybe first customers in 2021 ... then the resources would be available for Tesla to "fix what ain't broke."
We have an '18 P100D with the vented seats and upgraded MCU, and a '16 P90D with everything including the retractable wing ... I really like that wing ... : ) I'm going to try to update the P100D with that wing from the parts department. Is a used '16 X worth its market price? Yes. Is a new or used '18 P100D "worth it" ... hell no ... good grief, it's $150K+ ... basically $200K with CA taxes to get it depreciating on your balance sheet. But it's still a ****ing amazing daily driver. Wait? No.
My recommendation would be to get any X with a clean service history and not one with a stack of problems in its records. There have been some real lemons. Our '16 has had zero defects from the factory (perfect paint, wing doors never malfunctioned, no leaks, no gremlins) and each of the problems have been nuisances, not nightmares -- it has never failed to answer the key and go to the destination, never left us on the side of the road, never failed to impress us as an entirely new level of personal transport.
Every day I spent in a "luxury" Mercedes GL AMG or Porsche Cayenne Turbo S was a day lost. Sure, I enjoyed them in their day, but I just plain didn't know any better. I went to the very first customer ride-a-long of the Roadster and thought "I'll wait." What an idiot. We all stood around and said "wow ... but it's a Lotus with arc welder cables and a washing machine motor hanging out the back ... and the cabin looks like my nephew made it with his neighbor's sewing machine and some hot-melt glue ...." What mistake to miss out on that experience. I had one of the first batch of Model S's and decided "nope" (mistake) and the first batch of Model S with the first ever mention of Ludicrous and "sports" seats ... I passed on that one, too, though I think I probably made the right call because it would be another 6 to 9 months before that car finally worked as advertised ... I was an early-adopter who tended to wait, but now I'm just an early adopter (and looking forward to the Model 3 Performance (due to me in August) and the Roadster 2.0 (ordered during the unveiling) ... not waiting again!
I've lived with a 2016 X for 40K+ miles and I know what it's missing (360 surround parking cameras, range while towing, Android Auto, real ventilated seats, wiper/washer on the backup camera, removable 3rd row seats in the 6-seater, comfort mode in the air suspension) and it's 1000-2000lbs too heavy, it's too wide for city driving and parking, Autopilot has been turned into a nagging nanny, it doesn't have AEB or blind spot alert, and the body panel alignment around the wing doors is kit-car quality) but to repeat a cliche of thankfulness repeated by many others before me, a Tesla is like my wife: I'm not worthy of such good fortune and her shortcomings like the shortcomings of the X are invisible to me. : )
