Sand might be beneficial but I'm hesitant to add anything that is going to increase weight. If you have a clever way to move those 20 gallon buggers without killing your back, maybe it's not a big deal to you.
I think in a 20 gallon pot you will eventually get roots that grow into the ground too, maybe not in the current season if they were just up-potted, but certainly in the subsequent season. A 20 gallon pot filled with heavy mix and sand, with a bunch of 0.5 inch + diameter roots is going to be a project to extract from the yard and put in the garage, methinks. I am always scrambling around that time of year because there's an early hard freeze and all my trees need to be moved in 3 hours.
I'm sure it's a good thing when the roots grow into the ground, but it sure is a pain to move the pots come winter. Not sure if it's worth the headache.
On perched water table topic, also wanted to point out that I think if you have any amount of roots growing into the soil below, that means your potting mix in your pot is in sufficient contact with the soil below to wick away any PWT. The PWT only happens when wicking action is obstructed. So any time you were worried about PWT, you could just bury the pot an inch deep and the soil would wick away your problems. I even think if you place your pot on a concrete surface such that your potting mix comes into contact with the concrete that you would not have a PWT. Concrete will wick water, albeit slowly. Only if you place your pot on a surface that is watertight, such as plastic, or if the holes are not in contact with the concrete/dirt at all, then you will have a PWT.