It is all about setup/tune and efficiency. Just because a 281 makes more power with less cubes than engines of much larger size of yesterday - should that make it automatically better? There are too many factors that can effect such things with boost involved. Who's to say a different A/F ratio was used, different intake, larger valves, better breathing camshaft, better timing retard and advance implementation, more efficient turbo, a turbo that had a lot less lag so the power could come on sooner and spool up longer, higher or lower compression, different octane requirements, higher boost levels, or better yet - a design that was so poor by itself that haveing two turbos actually hurt performance because one could not satisfy the other to work in its optimum boost band?
There are too many factors that could have accounted for the change. what one has to do is take the exact same vehicle, year, tranny, driver, conditions, engine, ect. and compare them side by side then say what came out ahead. That's like saying because Ford went to .4 liters less, they got 75 more hp in return (92' 5.0 versus 05' 4.6) In order to make twin turbos work best, the engineers must make the turbines/impellers of the two turbos so precisely matched that right where one begins to just slightly drop off, the larger one seriously kicks in. Did you ever consider it was poor design by Toyota and that they made a much better turbocharger and it would save themselves some money as well to only use one instead of two? All variables I say.
On a last note, this is a 5.0 forum, therefore I was talking in relation and for a 5.0 application. I could, with all due repspect, care less about a supra or some other 6 or even 4 cylinder vehicles - those are different engines with different needs and work much differently than a torquey v8 performs. Each power adder has it's purpose and application, including nitrous. For each application though comes different needs, and it hugely depends as to which method would be of most benefit and yield the most power out of the other choices.