I have reason to believe that Apple is going to do a stealth upgrade to the existing Pros this Oct/Nov and bump the 512GB models to 6GB RAM.Honestly a just solid 4GB of ram available is plenty! You definitely could never use all of 6GB even if you tried. The problem with the 4GB iPads is, they really only have 3.69GB available before it is even turned on. After iOS is up and running you’ve got around 2.8GB available. So it can all be used up pretty easily.
So, I can assure you 6GB is overkill and more than enough in a iPad Pro.