I certainly relate to point 3: in a dead push Satanix isn't great, the ratio is a tad too low, but when I'm under something, it becomes a different ball game. Satanix 1 was a drill powered robot, only weighed 6.8kg, couldn't push jack. Then I tried it with Pain in the Asp sat on the front wedges (didn't have sharp ones at the time) and I carried him across the arena at quite a pace!

Also, there are ways to increase downforce but no up the weight, however in feathers I am not sure it's legal...

In ants, EDF fans have started becoming popular to increase the downforce on a robot significantly (The first one, FAntom, weighed in at 150g, turned on the fan, went up to 450g). It increased the power a lot as well (couldn't push a 1kg bag of flour, turns on the fan, pushes it easy). It also allows it to drive on the ceiling, just to show off :P.

Magnets are the other way, but you would need a: a significant drive system/speedo to start with to shift the excess, and a non-ferrous base plate that won't flex (Mouldy may correct me, but I think 360 or 540's base flexed out with his magnets, and also his Scorpion didn't like it.