If your performance is limited by your GTX1070, and not the cpu, then an upgrade is in order.
sli will give you impressive benchmarks, but gameplay will be better with a single great card.
SLI is prone to stuttering and screen tearing. Also, some games do not support sli.
If you need an upgrade today, GTX1080ti will be a good one.
No telling what vega/volta will bring, when it will come, what the performance will be, and how much it will cost.
Usually, a truly big jump in performance over current offerings will come at an exceptionally high price.