I mean, I could pretty easily get a really decent gaming experince on R5 1600 overclocked paired with 1080ti and there would be no bottle-necking whatsoever, not to mention with really kewl performance that is on par with I7 7700k, well in some games that is, but still R5 1600 is good enough for 4k for sure.
btw FormFactor is pretty limiting and it would be interesting to hear your reasoning for it, I mean ITX for 1080gtx, 16gb of ram and anyway R5 works perfectly with no problem...idk having a small FormFactor never been a popular option for many, and I think it will never be...for its lacks of features
Edit: ops sorry it seems that I kinda misinterpreted your post, sorry for dat.