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I found that when I stop the constant chatter (come on, lets go, yeah yeah, etc.) and instead ran quietly, ... works to give hima boost - I can actually see him speed up on video when I do this. \

As I'm sure you already know, the key terms you're contrasting there are "constant chatter" and "occasional whoop". ;D I, too, have found that an occasional exclamation can speed the dog up for my guys, I think it gets used most often when we head down a closing sequence, or hit an open part of the course after a tricky bit.
Now, with Morag, there's an additional and very interesting phenomenon: she's learned what the gamble whistle means, and almost always puts on a burst of speed when she hears it. Even more interesting, I've noticed lately that she sometimes seems to be anticipating it- IOW she's becoming aware of when we're getting close to the end of the 40 seconds. I'm sure she's made associations because of my reactions to both things, but it's interesting (and useful) none the less. :-)