While observing with my 20" tonight, a colleague of mine and I noticed a very, very good seeing and we decided to try and see how high we could push the magnification. On Epsilon Lyrae we achieved a magnification of 2560x by using a 4-2 mm Nagler click zoom eyepiece (2 mm setting) and a 2x barlow. Using the same eyepiece (4 mm setting) and the barlow, resulting in a magnification of 1260x, we observed the Ring nebula, which didn't fit completely in the field of view anymore!
Has anyone here tried such extreme magnifications as well?