I think you will find, that in SOME games, a single 1070 will perform better. In other games, 1070 SLI will perform better, but not by very much. And other games, that have good multicard optimization, 1070 SLI will scale very well and offer substantial performance gains, but the number of games like that are going to be few and far between because single cards have increased performance to the extent that most software developers are not actively trying to support multicard configurations anymore.
Would I go to a 1080 if you already have two 1070's? No.
Would I recommend a 2070 or 2080 over those cards? Yes, because you can probably sell both of those cards and make up most if not all of the cost of buying a single 2080 or 2080 TI. Even if you only got 250-300 bucks each for those cards, you'd have a small investment leftover to make in order to get a much better, newer card that is optimized well in current games.