Yap, originally I have ordered a single GTX1080 solution on D9X. However was told by Prince J that the system became unstable with 1 GPU and need to wait for another GPU with different ID to come in which will take another week or so. I hate waiting and that is why I decided to switch to GTX1070 SLI. The second reason is running single GPU in SLI enable board might lead to instability.
Anyway, I am happy with my decision. This machine is a monster. Tested with DeusEx Human divided. My old RX17 with a pair of GTX970Ms can't even hold the high setting consistently with some setting off while this monster is able to burn thru. it with Ultra, everything on. Min42 while Max at 64fps. This is a really impressive fps.
Also tested with Dumbuster's Homefront revolution with CryEngine 3. This machine able to run on it's highest setting with no hesitation at all. Silky smooth. On my old RX17, I have to run this game with medium and have to disable a few selection to get the game on acceptable fps.
If you have the budget to spare, I would recommend you to go for SLI.

Here is my spec.
17.3" Full HD Anti-glare Display featuring 120Hz Refresh Rate (1920 x 1080)
Professional Color Calibration
Intel Core i7-6700 DT processor
2 x Nvidia GTX1070 8GB DDR5 Pascal
IC Diamond Thermal Compound
Crucial 32GB Dual Channel DDR4 RAM 2133MHz - 4 X 8GB
1TB 7200rpm Hitachi Hard Drive
120GB Intel 540 M.2 SATA Solid State Drive
Killer Wireless AC 1535 M.2
Win10 Home 64bit
ILLEGEAR 2 Year Carry-In Warranty

And here are some of my benchmark:

Time spy:
New rig with a pair of GTX1070 manage a score of 8446 ( http://www.3dmark.com/spy/520185 )
My old rig with GTX 970M only manage 4272 ( http://www.3dmark.com/spy/34254)

Fire Strike Extreme:
12938 on new D9X ( http://www.3dmark.com/fs/10318196 ) while on my old rig is only manage to chuck out 10802 in normal, non-extreme test ( http://www.3dmark.com/fs/6189391 ).

Cloud gate:
27538 on my new D9X ( http://www.3dmark.com/cg/3802140 ) while RX17 manage to pull out 23540 ( http://www.3dmark.com/cg/3100725 ) which is not bad. This test do not scale very well with SLI i think.

Heat is definitely not an issue for this machine. Never see the temperature go above 70C for CPU and the GPU hit the max is 82C (when running Dumbuster's Homefront for about 20min on the highest setting). Dumbuster's Homefront is really a system resource  guzzler which incidentally running on CryEngine3 and it is a very beautiful game. This is the only game that turn the system fan up a notch so far beside futuremark benching.
I have tested it with Batman Arkham Knight, GTA V, Doom, TitanQuest Anniversary Edition(crunching this game now), DeusEx Human Divided. None of this post a challenge to this machine. It just burn right thru. them with ease. Never encountered any micro stutter or throttling at all. I don't think there are any game will make this machine struggle yet as of now or maybe I am just gaming in 1080p. Twin GTX10170 are just overkill on the resolution that I play. I have pre-ordered Gear 4 Ultimate edition from MS Store, let wait and see how this machine perform on this new DirectX12 title. Heat is never an issue for this machine.

Sound is also very good compare to my old RX17. Very solid and vibrant with depth.

I will try again tonight with DirectX12 game such as Forza Horizon 3 (just finished downloading last night), Gear of War Ultimate Edition, Recore and the dreaded Quantum Break from MS store. Will update here again later.

And lastly, I have to agree with your concern in SLI supports. Almost all the game being release without SLI support. They normal came a bit later with nvidia driver update. Once the game being supported, they fly. There are always a alternate solution to title which didn't support SLI. You can use the Nvidia Inspector to tailor you own SLI support or download the profile from fan's forum.

Have been using SLI since RX17, I am very confident that SLI are here to stay. Especially when people want to push 4K now. Single card barely cut it.

The reason is because my son's 4yrs+ Alienware M17xR4 starts to give problem now and he is hogging my RX17 for Overwatch. I have no choice but to get another machine for myself.