Have you used any non-reference Vega cards? I'm curious because I do not have a reference card. I have an ASUS ROG OC 64, MSI Air Boost OC 64, & Sapphire Nitro+ OC 64. Hashing power is about the same on all three, while the ASUS does a little better on power consumption. I would be curious how the reference cards compare on power consumption.
I've not used any no, non-reference should run cooler, so many the gpu die would overclock better. But generally your hashrate will be identical to mine if you run your gpu core at the same speed. The only question would be if you would be stable at that speed at the same voltage level.