Jeezus, yeah id be going for the 2700X and using that extra money for a upgraded GPU, for $200-250 more in the budget for a GPU, which one do you think games better? I know where id place my bets...
If you were to go with a 2700 instead, its a full $300 price difference lol. Huge jump in GPU performance at that level of pricing disparity, allowing you to go from a 580 to Vega 64 or 1080 to 1080ti
If pricing were closer I can see the CPU performance uplift might make sense, but from what it looks like currently, the 9900k is a definite pass.
At 1080p you probably would see a difference, but anything above that gets pretty even. The Intel is still going to have a slight fps advantage, but unless you're going for 144hz it won't be noticeable. You're probably right in the fact that the $300ish going toward a better gpu would probably end up being the way to go. That puts you in a whole new tier of cards.