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All of that is true--vignetting is very minor, however.
The lengthening of the focal length can be calculated--for every 1mm you add at the back, you increase the scope's focal length by 3.1mm.
The main issue with the 8SE, however, is not having enough clearance for the 2" diagonal to pass through the bottom.
This can be avoided by, say, using a Baader Dielectric diagonal, whose nosepiece can be removed from the diagonal, leaving a thread in the body of the diagonal that can be screwed directly onto the back of the scope.
You need a small ring Baader sells to go between the diagonal and the scope so you can lock the diagonal in place. You add the least back length that way, and add the least focal length to the scope.
Also, the optical tube can be slid forward in the cradle to give more clearance for the diagonal on the back. For many 2" diagonals on the 8SE, you have to use a Dremel tool to cut away a bit of the plastic covering the fork arm to allow the scope to slide forward more so the 2" diagonal will clear the battery department cover when it swings through.
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