Currently trying to take over an NPR homeworld. They have about 120,000 tons of ground troops defending it (mixed tech levels from Infantry -> Heavy Infantry (which I assume is Power Armor), but mostly low level. I attacked with 420,000 tons of Heavy Power Armor, Super Heavy Tanks, and Artillery (so significant tech edge) but lost pretty badly. Tried again with 500,000 and made more headway, but still would have lost. Now I'm attempting two waves of ~500,000 tons each. Won but didn't have enough GSP to kill all of their infantry, so back to the drawing aboard again. No fighter or orbital support.
BTW, doing some quick testing, it looks like FFD has no effect on artillery.
Against Precursors, I tried 80,000 tons of PA infantry versus 20,000 tons and got totally stomped. Haven’t tried again.