You can do two things. Just stick with a normal 1070/Polaris for now and do your glass tubing when the big boys are released or just go 1080 now as that is the best there is now.
My perception is the 1080 performance is still like what 28nm chip released now would offer.
The only reason I would say do the former is because we have not seen the boost this new node has to bring yet. When that big boost kicks in with the new cards, you may be left feeling bad.
Hope this helps. Whatever you decide, enjoy