I certainly do not disagree that Yamaha makes good sounding boards. I'm running one right now (a PM5D) for a show I'm on where I'm mixing live opera outdoors- a full orchestra, soloists, chorus, etc...
What I would suggest to the poster here is to get a good clock- either a rental or a demo. I mentioned the couple of clocks that I use most often. Connect it to the clock input and make sure the termination is switched on. Use your ears and decide if you like it or not.
I don't have scientific figures, but I do know what I've listened to. As a results of my listening tests, I won't mix on the consoles without a quality external clock- it has much more to do than just system integration (although when I use multiple sets of converts, and recorders, it is a lot easier to just run everything off an external clock).
--Ben