Yeah don't get SLI, you can't rely on it working at all and an 880M is basically twice as fast as an 860M in raw performance anyway, definitely a much better choice.
As @
Pilluminati said the very high end CPU's just aren't worth it in terms of price/performance, you're generally best off getting the lowest clock speed version of the best architecture, which right now means any i7-47xxxx.
Those Pcspecialist laptops are good bang for buck, not a bad choice for sure and seems like the best spec/value for a high end 15" but I'm not sure what the generic chassis will be like in terms of cooling/noise. The 880M will produce a similar amount of heat to Pillumati's M290X, so maybe he could give you an idea!
Seeing as you want a 17" screen anyway another good choice is the Asus G750JZ, it's a similar spec/price with great cooling. Ebuyer have a good price on an ex-display one that you could still get shipped today if you're feeling impatient:
http://www.ebuyer.com/665949-exdisplay-asus-g750jz-gaming-laptop-intel-core-i7-4700hq-2-4ghz-16gb-ram-1tb-ebr2-g750jz-t4056h
If you want to go with the Pcspecialist one I'd get the Vortex IV 17 base (
http://www.pcspecialist.co.uk/notebooks/vortexIV-17/) and spec it like this:
i7-4710MQ
16GB RAM
GTX 880M
1 or 2TB HD + 250GB Samsung EVO SSD
MX-4 thermal paste
Killer NIC (Bluetooth & Wireless)
No OS