I don't think your going to get a really dynamic answer as the end user isn't really doing these types of complex analysis on the exhaust and personally I wouldn't trust a manufacturers opinion because they are obviously biased.
From my understanding of the basics, a dual y-pipe is just to equal out the backpressure from each side of the engine. I am honestly not sure why a company would go from a dual into a single midpipe back into a dual, however if your diameter is big enough on the midpipe it probably won't impact performance dramatically (might even increase low end over a dual). I wouldn't worry about air flow or turbulence because the majority of the turbulence would be at the begging of the Y when the gasses meet and equalize.
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