I have a 650b, 29 and 26 bikes and I would say that more than wheel size, overall geometry plays the most important part. The 29'er with its 71 deg HA is a wicked fast cornering bike and is fast, the 650b HT is slower feeling, but is has a 65 deg HA (On-One 45650b) and the 26 inch bike shreds the downs better than the others, but it is a TRc. Horses for courses. But, the one thing they all are is fun!
For sure there are factors other than just wheel size at play with my experiences to date moving to 650B… bottom line is I have no regrets making the switch and any perceived gain in performance is still a positive.