When I change memory freq using any tool (WattTool) for a single card while miner is running, it instantly changes displayed hashrate only if the card has connected monitor (tested on Rx470). For headless cards the hashrate shown is not changed until miner restarted or card stopped/resumed. That seems to be a feature (@All: please confirm, but I saw someone complained for the same issue).
When you change freq using miner cmdline, and previous freq was not the same, miner sets freq but shows hashrate for old freq until restarted.
I was using -cclock for a while - it works good. (in fact it's the fastest way to identify cards if you set something like 1010, 1020, 1030, etc...)
Watttool would apply values immediately if you uninstall the AMD drivers crap and just point devices to the driver dir.