TBH Ive been considering the Sigma 150-500 myself but would it give me enough real advantage over adding a 500mm Tamron mirror lens to my present setup?
Arent mirror lenses notorious for being a bit ropey?
Most are ropey, the Tamron ones are probably the best. They are all limited to F8 though and anything out of focus appears as a donut ring rather than a splodge. On the worst quality ones this ring can be oval.
Really it depends on the quality of the mirrors. At the other end of the scale you end up with massive telescopes like NASA Space Telescope, I doubt if anyone could claim that is ropey*.
*at least since it was fixored :mutley: