Tele Vue Powermates are available in 2x, 2.5x, 4x and 5x with the 5x unit capable of 7.7x when imaging.
See
http://www.televue.com/engine/page.asp?ID=237
and
http://www.televue.com/engine/page.asp?ID=42
No need for eyepiece projection as in the past.
eg. 2000mm f10 C8 will give 15400mm at f77. C9.25 perhaps even better choice since secondary amplification ratio is less than in C8 and C9.25 tube fits nicely on GM-8 and has manageable weight. Note fork tubes will not give clearance for long accessories.
A 2.5x or 4x Powermate may be more suitable more often depending on seeing.
Good tracking highly recommended due to high magnification and small imaging sensor size.
Whatever you choose, enjoy!