Mark,
1) I was incorrect in quoting Russ as the author of the F-ratio myth...it was in fact Stan Moore. Interested punters can read about it here:
http://www.stanmooreastro.com/f_ratio_myth.htm
Do you disagree with Stan's analysis?
2) You are assuming the primary is decentered by the factory. I've tested close to a dozen of these scopes and not one had a problem. Also last time I checked an F5 of anything was far more of a bugger to collimate than a F8
3) The point Greg made was the *tube was longer*.
It is patently not,
and will suffer less from wind loading as a result....though I do take the point the longer focal length could be more sensitive to seeing and tracking errors.