Another way you could look at your statement is, for a larger engine, they simply don't make turbos large enough to give you the same situation you have with your smaller engine example. The displacement and breathing capacity of engines that people are slapping turbos on has increased at a faster rate than the development of turbos that can efficiently support such engines in a single turbo configuration. That doesn't mean the principle is not correct. If a turbo was developed to be run in an efficient range on a large displacement engine in a single configuration, it would kick rearend. But the market for such a turbo would be very small. For one thing, how would you mount such a monster. What would the exhaust pipe size need to be with such a single turbo configuration?The only scenario where this applies is where the engine is small enough that an extra large single can be sized to keep the backpressure at 1:1 or under which is where twins would operate.