I had an Istar Perseus 150mm F10 mounted on my AZ EG6GT. That thing was massive! The mount coped ok, the sheer weight of the scope contributing to the stablity of the set-up. Alas, my back was less happy with having to mount/dismount the scope every session, so I found it got increasinlgy limited use until eventually I sold it. I really miss the views it gave me, but unless I can keep a permanent setup, I reckon 120 - 130mm is about as big as I can go in refractors.
I still hear the siren calls of the 140mm APMs though...