Say, any word on how the plane is flying now?

The main problem is that the battery configuration is wrong. Using glow engine terms, you hook batteries in series to increase "cubic inches" and hook in parallel to increase the size of the "fuel tank."

Two 3S4P packs connected in parallel give you a really small engine with a huge fuel tank. You want them in SERIES, to make a 6S4P configuration. With the right prop, that setup can easily fly most .60-size sport planes on the market.

I'm wondering, how was this power system conceived? It's serious overkill for a .25-size Piper Cub. There's at least a pound of weight and a couple hundred dollars that doesn't need to be there.

That plane would fly extremely well, for 15+ minutes, on an AXi 2820/10, using an 11x7 prop and a Phoenix 45 ESC, with only one of those packs.