Maaaa I dunno..?
It is not the focal length exactly really but more the fact that if the long focal length is provided by a multi mirrored telescope with truss tubes, inadequate focusers, loose mirror mounts or with floppy mirrors, ie Newt, RC or SCT there is potential for differencial movement over time. The significance of this differential movement is a function of the focal length and your image scale sure but the degree of flexure and the time period over which this movement takes place will depend strongly on the class and design of your telescope and guide scope interface.
It is clear from the above comparisons that self guide is simply not necessary just becasue you have a long focal length, the comparisons show rather difinitively that for 10 - 15min exposures at least, the absolute resolution possible without self guide is clearly equal to that with self guide and even rivales AO, atleast for modest exposure times and with a good quality mount.
Due to the limitaions of self guide (poor transmission through filters and faint or no guide stars etc) there are many imagers (most?) both long focal length and short, with selfguide cameras, who are basically forced (happily) to use piggyback guiding and they acheive excellent results!...why worry about having to have the self guide if piggyback works just as well and you must have a guide scope anyway? this is why I think the self guide feature for most situations is just simply overated.
Don't let anyone tell you that darks will always take care of the noise level in your noisey warm subs either, this is not true, excellent cooling does make a difference this is why cameras with poorer cooling cababilities very often have to resort to messy cumbersome water cooling, and others even resort to putting their cameras in a fridge
while imaging
.
Self guide is a feature for sure but there are other qualities in a good CCD camera that I believe deserve more consideration - namely: overall noise levels, cooling abilities, download speed, method of keeping moistier at bay, build quality and reliability.
Mike