Well, there's always clock chipping... I don't know how much head room the processors in the 1.42GHz version have, nor how efficient the copper heat sink is. I have purchase four or five beefy heat sinks with heat pipes all over them from the PC oriented world, and one of these days I will find a circular tuit. Then I'll cobble some adapters to mount these heat sinks in the MDD (all chosen so their bases will cover dual G4s, and their height won't bump the hard drive mount) and test who provides the best cooling. Following that, serious overclocking to see how fast the MDD can go with stock CPUs and serious cooling.
Of course, the 1.42 GHz copper heat sink may already be as efficient as the specialized PC heat sinks, and the G4s may already be near the top of their performance ceiling...
I think there's a web site where some fellow took his MDD dual up close to 1.6 GHz. Since you won't get much bettter than 1.8 GHz from upgrades (IIRC) that's pretty good.