If that is the 2008 ZX10, I wonder why they opted to revert back to the single exhaust instead of the dual undertail ones from the '06-'07? Do you think Kawasaki determined that the benefits of the dual exhaust (release more exhaust gases easily) didn't out weigh the added weight the dual exhaust pipes added to the bike? :headscratch
Two pipes do have a bit of a weight disadvantage but then again, maybe they have one on each side as the K7 Suzuki. i hope not though. I am a fan of the single side pipe. I honestly wonder what the new one will look like
My guess is that the two pipe thing on coming out on the new litre bikes is to keep the sound down. With two pipes you can flow more air (as stated above) and thus make each pipe more restrictive (to limit sound). The end result is more hp from a stock system with more weight. They have to keep the emissions guys happy yet have good hp #s to sell the bike.
Just my take on it.