8 + 6 is a nightmare to power in a multi-card rig compared to just 8-pin, and you're not going to push any Polaris card hard enough to NEED more than an 8-pin when mining.
8 + 6 will NOT ease the power load from the risers - the intent is to allow massive overclocking by gamers.
some card work if only 8-pin is connected (Sapphire RX 570 Nitro+ 8GB), then it's ok. Others don't (MSI GTX 1080 Armour). That's what I personally know of.