G'day Hugh,
Nice to be hoovering up the photons at break neck speed!
I'd recommend the baader RCCi coma corrector
http://www.teleskop-express.de/shop/...-distance.html
setback is around 94mm to go longer I think you will need to go exotic. (The MPCC is only 55mm)
But then again will you need the AO in a shorter focal length scope? If finding a CC more than 100mm it might get expensive.
I think you will be pretty safe with a CF model in terms of worrying about tube deformation. I remember seeing a pic on the teleskop express website demonstrating the strength of the CF tubes think it had cement bags sitting on them not warping at all.
Collimation at f4 does take longer but nothing to worry too much about really.
The focuser upgrade is a must (but this is true of most scopes) - secondary mirror collimation screws replaced with bob's knobs (so you can adjust by hand) and you're on your way.
yep I don't know why more people don't use them, throw a barlow in and you've got good FL too for close ups.
Cheers
Russ
ps if considering gso you will need to replace secondary springs as well and maybe the spidervane. The skywatcher in my opinion is a higher quality build. if I had my time again I would pay for the SW over the gso (although they didn't have 12" f4 options so I didn't have much choice).