the dual stage and low vtec point helps maintain a broad powerband. by switching to a single runner manifold you shift the powerband up and make the engine peakier, taking away from the low and mid ranges.

when people start boosting or doing na builds they dont tend to focus on these parts of the powerband cause in most racing your not here for very long. they want to maximize the part of the powerband they will be in most of the time. also, by going to a bigger bore or more boost or any number of various engine setups you can effectively get some of that lost powerband back.