Looking at the potential for multiple GPUs to help, dual 980 Ti cards in SLI do get us well into the playable range, particularly if you're running a G-Sync display. Lack of VRAM may be part of the problem here, as a single R9 390 actually manages slightly better 97 percentile results, but drivers are almost certainly a big part of the problem. AMD's R9 Fury X meanwhile falls between those two in average fps, but the minimums are substantially worse, often dipping into the low teens. The GTX 980 is about 20 percent off the pace set by its big brother, averaging exactly 30 fps but with frequent dips into the low-to-mid 20s. A single 980 Ti at stock breaks the 30 fps mark, but not by much, with lows still dipping into the high 20s it's still playable, for the most part, but it's not an ideal solution.
We might see some performance improvements over the coming weeks, but we're far from breaking 60 fps at the Very High setting, and even using the High preset we're still coming up short on the fastest current GPUs. 4K can be punishing even in the best of situations, and Rise of the Tomb Raider is certainly not going to be a best-case scenario, particularly so soon after launch.