PhoenixMiner 3.5b works stable for my only 2x Sapphire RX580 Nitro+ rig but I don't see any change on my GPUs power draw when using new green kernels. I get same hashrate (30.3MH) with same GPU only power draw in GPU-Z sensors tab (90W for every card) when using new kernels.
Never use GPU-Z for power measurements, hardware watt meter is the only sure way. Everything else is just software estimate that is often pretty wrong