Hi Tony,
Interesting point of view. Firstly, I would recommend measuring the rear cell opening. A 3" focuser could possibly reduce vignetting, but also maybe over kill if the internal baffle opening is only 2" or 2.5" in diameter. I use an eyeopener 2" visual back and a JMI 2" focuser modified with a Robofocus stepper motor. Manual or computer controlled - it works a treat. Simply unless you intend to use a medium format film (6x4) I doubt you'd need something over 2". Even the SBIG STL11k large format CCD camera that has an equivalent frame size of a 35mm camera only uses a 2" nose peice.
Perhaps more importantly, is not the diameter, but the length of the focuser. SCT's have generous back focus, but not all telescopes do. If you imaging train i.e focal reducers, focuser, color filter wheel, camera is too long, i.e move far from the native (most optimal) position you may hit other issues. Just some food for thought.
