If you get a FTW 670 it won't 'become' a generic 670 just by running it in SLI with a generic card. It still has the better (GTX680-sized) PCB, better cooler, better power section, etc. It will also run at whatever clocks it runs at from the factory, it will NOT downclock to the slower cards clocks. It'll also (I'd bet) maintain it's value better.
However, the two cards together in SLI will in most cases perform the same as having two cards at the lower of the two clocks. E.G. a 1050MHz card + 1100MHz card in SLI will usually perform like two 1050MHz cards ... what ends up happening is the faster card 'waits' for the slower card. So you'll see the 1050MHz card running at 99% usage, and the 1100MHz card running at 95% usage or thereabouts.
But whether or not this happens may depend on the game you're running. Some games in that scenario may perform like two cards at 1075MHz ... the average of the two. I can't say for sure that will happen with 670's cause I've not tested it, but I have seen card reviews done that showed an 'averaging' of performance between the two 'mismatched' cards in certain games, so I know it's possible for that to occur.
However, no matter what, both cards can be OC'd separately though using PrecisionX. It's very likely the generic card could be OC'd to the speed of the stock FTW, or perhaps even higher. If it were me, I wouldn't hesitate to grab an FTW as your 2nd card for SLI