No, the Super Flower Leadex (EVGA G2/P2/T2) is among the best of the best. The only thing that beats it is the Corsair RMx, but only by a tiny bit. These are the very best out of all the PSUs on the market.
The EVGA P2 isn't any better than the G2, except it's just a teeny tiny bit more efficient. That's it. It's not worth spending $15 more on a 750W PSU in comparison to $100 on an 850W PSU when they are otherwise identical in terms of quality.
You said you might want to add another GTX 1070. That's still not going to justify getting a PSU that's any "bigger" than a good high-end 650W unit. I'm dead serious. The 6700K's power consumption is extremely low and the GTX 1070 is also extremely low. What kind of overclocking do you plan to do? I mean, with basic overclocking to both the CPU and both GTX 1070s (provided you ever actually go ahead and add another), a good quality-made 650W PSU would be far more than enough and it would cost you even less money on this build. For example:
$79.99, for a top-of-the-line 650W PSU. Why spend more when you don't have to? Even if you were to somehow manage to constantly pull 550-600W out of this PSU, you'll never make it beg for mercy. PSUs of this caliber are designed to be used like that and they're designed to last several years very easily.
If you still want a 750W PSU though, then it can still be had at a low price:
$99.99 after a rebate, $119.99 before it.
With two GTX 1070s in SLI though, your power consumption while gaming will ride 500-525W. Maybe lower. So, we're really looking at overkill PSUs here.
I'd also say that we're looking at overkill in quality because it's not like you're doing professionally competitive overclocking, aiming to win world championship competitions or something. You're just making a gaming rig.