A 15-20 reach will cause a lot of problems when I'm 15 feet directly above someone.
Tell me about it, meleers hate that Sturmherz used reach and flying above his enemy before, though he accomplished it with glide speed from dragonborn wings and insane jump+natural elevations to get the height. That concept will certainly work against many melee fighters, but you still need to win a fight for the potion first or hope they don't bring a reach weapon themselves

as for commoner + reach, my commoner has 20 feet without potions, though of course lacking the flight. I could have given him Starspawn instead of Deepspawn for unlimited average flight with landing every 4 turns or so. But I just liked the tentacles too much
Would be kinda funny if our two commoners run into each other in an official Round