The concept behind 1, 2, or 4 turbos, is not a discussion of peak HP but rather the shape of the power curve. A single turbo can be chosen for a GT that would yield the right boost level to achieve the desired (max) HP objective. The traditional drawback of a large, single turbo is the time that it takes to spool the larger turbo. Utilizing two turbochagers, each one can be smaller with a smaller (mass) impeller and therefore with quicker spool times, i.e. less lag and more boost sooner in the power curve. Well, if two is good, how about 4? The same concept of smaller mass turbos and faster spool times is valid. However, you reach the point of diminishing returns - especially when you starte to consider all of the plumbing additions and the potential failure points that you are adding.
