The FE cards are not the same as the non-reference cards. For one thing, every reference card out there is thermally throttling. The oft made statement that you can overclock reference card to non-reference cards is what is called a false equivalency. Yes, your chances are good but ignores the fact that the non reference card can be overclocked too ... more than the reference card and b) all the FE cards are throttling.
The 7xx and 9xx series cards saw no advantage from CLC type (GPU only) water cooling. The overclock here was oft limited by VRM temps and / or voltage. A full cover water block solves two of the three. The only way possible for the non reference PCBs not to matter is if nVidia voltage limits the cards and doesn't allow any higher voltages or power limits. And that's an answer we already have... the aftermarket cards do in fact allow you to set a higher power limit than on the FE cards.
MSI 1070 = 120%
FE 1070 = 112%
Obviously, the card which allows more power to be used, will habve an advantage in overclocking,
So no, water cooling an FE card will not offer you the equivalent performance difference that an aftermarket card will.
Which FE in particular, the only practical answer is the one whos vendor logo most appeals to you as that represents the ONLY difference between the cards. This generation, we have gone beyond silly CLC kits and you will be able to buy non-reference cards with full cover EK water blocks re-assembled.
http://www.tomshardware.com/news/msi-ekwb-watercooled-1080-1070,32104.html
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127952
At an $80 price premium over the air cooled MSI 1080 Gaming, it presents the proverbial "no brainer" with retail costs for the WB averaging $130 and an additional $30 for the backplate in recent years. Looking at $700 for an FE card, the water block / backplate combo typically adding $150-$160... the premium PCB Seahawk is still cheaper....and a) ou don't have to spend another $10 for TIM, bZ) don't ahve to do the labor and c) the entire assembly comes with a warranty.