Hi Guys,
I have a 10" GSO Dob, and had the same problem with a canon 40D. Since then I have installed the GSO low profile focuser and using an MPCC have about 2mm of spare focus if that - This was also achieved with moving the mirror up slightly on its collimation screws also.
Its a close call but it can be done without modifying the tube
EDIT: Im also running this scope on an EQ6 with guide scope and have had pretty good success with it.