While styrofoam is obviously bad, you say - wad will scrape unburnt powder, while grease will lubricate.
However, when you press the trigger, the first thing to go in the barrel is the grease. The the bullet comes, which must scrape most of the grease out if it is a good fit (if it is not a good fit to the barrel, forget accuracy anyway). Then comes the wad which scrapes the rest of the grease if it's not lubricated, and then comes the actual powder residue. Basically, every shot except first comes out of a fouled barrel. This is more or less necessarily true. At least, though, if the grease stays in front of every bullet in the chamber, it will ease the passage of the bullet, but grease in front of the ball sometimes gets blown away or melted by previous shots.
A lubricated felt wad or hard grease between the wad and the ball could at least make this powder residue softer, meaning less of an impediment to the next bullet. This is at least my logic.