Telephotography experience

[bear]

Currently my longest lens is a sigma 100-300 with 1.4TC on a Nikon D300 (small sensor) (equivalent to 630mm on full sized sensor). Going longer gets very expensive with Nikon lenses. I'm wondering about an Opticron scope with T mount adapter to get quite a bit longer. Any photographers here with telephotography experience, particularly image quality compared with my 'budget' set up?