I'm setting a personal longest post ever,:badidea: so here goes:
There are a lot of factors that come in to deciding how much cfm vs. boost at a reasonable temp. you need on these trucks. The greater amount of fuel you can efficiently burn, the more power you can make. To efficiently burn X amount of fuel across the RPM range, you need turbo efficiency the whole way through. Just adding turbos to show how much of a tough guy you can be is stupid IMO. I don't see anyone on this site doing this, but the technology behind the design is much more important than the quantity of turbos, and staging setup.
Turbo and super chargers have a compressor map, quantifying where the chargers efficiently move air, and thus make power at certain pressure ratios. The 6.4 (and all the other power strokes for that matter) have pretty crappy flowing heads, manifolds, etc in comparison to any turbo'd/non-turbo'd gas engine of comparable power, so we force the air in faster by cranking up the pressure. Of course, other things can be done to increase CFM, and without raising boost pressure by doing the proper cam, ported heads, manifolds, etc, but as of now, there's not enough technology to get the head flow to where we can do 1000 HP on low boost.
Parallel twins are more efficient than a single using identical technology, sized equal to the two parallel chargers combined because of efficiency, centrifugal force, and probably a handful of other factors. I'm sure there's a ton of info on the web comparing the two for those of you interested in doing parallels, but it's not my area of expertise. The downfall to parallel twins in the diesel setup is that when space is limited, and you can only have 2 turbos for example, you want the turbos in a compound configuration making more pressure than the parallels can to overcome the comparably crappy head flow.
Compounds allow multiple turbos to cover a greater rpm range efficiently in a higher pressure configuration by allowing smaller chargers to remain in their map range at higher pressures because they're being force fed by larger chargers. The max power compounds we offer can make 1000+ fuel only HP with comparably amazing results, and great streetability for a handful reasons that I can describe here. Godzilla's setup is a similar setup, but force-fed by a 106 mm charger, making things more efficient in the higher rpm/power range.
> The high pressure turbo has a reasonably sized compressor for off-the-line power in street driving situations, with the VGT allowing the charger to light faster than equally sized non-vgt turbos, while still allowing comparably more exhaust CFM past the (larger than stock) turbine wheel, and dumping excess back pressure to the next turbo once it's moving a lot of air because of the huge turbine housing.
> Properly sized atmospheric charger matching the high pressure VGT. Before the VGT is beyond it's map range, the atmospheric turbo is force-feeding it, allowing it to stay within the map range, and increase manifold pressure, thus increasing CFM past the crappy heads, which really matters.
Godzilla's triples work off the same principle, but have yet another turbo force-feeding both of the other chargers, thereby allowing pressure and CFM to further increase efficiently, and therefore make power beyond the high power X (62/82) base setup.
Here's a pressure example:
VGT/total pressure is
105 boost/105 BP
BP falls off as it goes into the 88 mm intermediate turbo, which runs at
70 boost/70 BP
All of this ends with the atmospheric turbo (106 mm) producing
35 boost/ 35 BP
Atmospheric turbo is generating 35 psi of boost at 1:1 ratio, intermediate turbo is making 35 psi boost at 1:1 ratio (70-35=35), and the high pressure turbo is making 35 psi of boost as well (105-70=35), equally splitting the work to efficiently allow 105 psi of boost, and still have low rpm, off idle power.
This is only an example, of course for explanation purposes, and there are a lot of other factors that come into play in making this configuration work properly. That's the idea behind the design we're using on Godzilla, and our other triple compound setups.