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Reason why I also included the 980ti was I hoping secondhand ones will drop to less then £250 in a few weeks time with seeing the new prices on them dropping like a stone...Go 1070 rather than the Ti imo. Go forward, not back
I Never really had a problem with driver support for older NVidia GPU'sI'd rather have driver support
Posted some benchmarks a while back of 1080 and Titan X in a few games. so hope this provides some information. Most games are Ultra / highest, so notch a few settings down and a single 1080 is happy with a 3440 x 1440 panel.
Titan X Settings
Stock Settings - 1316 Mhz core / 7012 Memory throughout tests
Overclock Setting - 1474 Mhz core / 8020 Memory throughout tests
1080
Stock Settings - 1848 Mhz core / 10012 Memory throughout tests
Overclock Setting - 2101 Mhz core / 11030 Memory throughout tests
All tests were at 3440 x 1440. Scores below are average fps with Min / Max in brackets, though for some tests the Max / Min are pretty crap representations.
Far Cry Primal - Ultra Preset + HD texture Pack on
Titan X Stock - 53 fps (45/59)
Titan X OC'd - 60 fps (52/67)
1080 Stock - 61 fps (54/71)
1080 OC'd - 68 fps (59/78)
Arkham Knight - Maxed including Nvidia features
Titan X Stock - 63 fps (47/89)
Titan X OC'd - 68 fps (48/98)
1080 Stock - 80 fps (43/120)
1080 OC'd - 94 fps (75/136)
Checked this game 3 times as that's a pretty large and unexpected jump, but results are correct!
Tomb Raider - Ultimate preset
Titan X Stock - 88 fps (70/110)
Titan X OC'd - 99 fps (73/122)
1080 Stock - 97 fps (70/116)
1080 OC'd - 104 fps (80/128)
Shadow Of Mordor - Ultra Preset
Titan X Stock - 83 fps (55/150)
Titan X OC'd - 93 fps (48/141)
1080 Stock - 95 fps (52/181)
1080 OC'd - 105 fps (49/277)
Metro Last Light - All maxed including MSAA - Motion blur
Titan X Stock - 34 fps (10/130)
Titan X OC'd - 39 fps (12/162)
1080 Stock - 38 fps (20/78)
1080 OC'd - 42 fps (14/82)
(Maxed everything on this game including advanced PhysX aside from Blur, for real world some stupid settings would be toned down)
GTA 5 - All settings maxed minus Water MSAA and general MSAA
Titan X Stock - 69 fps (26/147)
Titan X OC'd - 81 fps (31/134)
1080 Stock - 81 fps (23/150)
1080 OC'd - 87 fps (31/130)
Hitman Absolution - Ultra preset + 2 x MSAA
Titan X Stock - 68 fps (62/105)
Titan X OC'd - 78 fps (66/92)
1080 Stock - 85 fps (72/104)
1080 OC'd - 95 fps (78/114)
Dragon Age Inquisition - Ultra Preset
Titan X Stock - 61 fps (50/87)
Titan X OC'd - 68 fps (54/96)
1080 Stock - 73 fps (68/114)
1080 OC'd - 80 fps (68/118)
Witcher 3 - Hierarch square (same spot with NPC's) - all settings maxed including hairworks + hairworks AA x 8
Titan X Stock - 50
Titan X OC'd - 53
1080 Stock - 55
1080 OC'd - 57
I have a 980ti and x34, it's a good combination and the card copes well. You have to turn a few options down in some games, but nothing to drastic. I imagine that the 1080 is great at that resolution. Tempted to get one myself, but I think I'm going to wait a while as I'm not really struggling with anything atm.
I wonder if any of those 1080 results are CPU bottlenecked. Maybe try adding another 2x anti aliasing to rule that out. But maybe they are not anyway, although some of the 1080 results might be limited by CPU. You have a very nice CPU, but the problem is a lot of games do not use it properly, hopefully that will improve with DX12.