I can attest to Wax's assessment they run much better with twin carbs and without the reed stuffers , I ran dual 38 BN carbs with a Hot Products manifold from day one on mine since it was built from New parts , it always ran really well .
somewhere down the line I ran across a Westcoast dual manifold that had the dual pattern for Mikunis and Kehins on it, I also ran across a dual Kehin setup for a Yami 650 , I used the dual Kehins on the Westcoast manifold and removed the reed stuffers , the increase in throttle response and power was amazing. I can only guess that with dual SBN 68'S or 44'S with the right setup it would have been even better.
These 550 reeds really haul if they are set up right , I really wish I had kept my setup in my 300SX and and run it with the 650 pump , I was also running the Westcoast dual total loss setup on that engine. and a Coffman half pipe .
I also did a 44SBN conversion on a customers ski , it did run really well but not as well as the twin carb setup did.