There is one other thing that REALLY helps with dobs at high magnification - fine tuning your mount!
I have no problem using my 8" f/4 dob at 320X. Even when using an eyepiece with a modest 58° apparent field of view. I made a series of sketches of a shadow transit going across Jupiter a little while back, and I only used my manual 8" f/4 dob for this at 320X, and not the 8" SCT with tracking I had at the time for this. The whole shadow transit event lasted for two hours, and it was pee easy following Jupiter all this time. I could have used that SCT, but that dobbie is just so much easier to set up and I was just too lazy arsed to set up the SCT, but I knew I could do this just as easily with this dobbie. The trick though is keeping the mount itself tuned up.
Below is the little video I made using that series of Jupiter sketches of the shadow transit. I didn't use any colour filters, just a neat view of Jupiter using a 5mm 58° eyepiece in that 8" f/4 dob.
Tuning up a dob mount means giving it a little TLC at least once a year so that the bearings are moving nice and smooth, and the scope is not sticking or difficult to move. As the bearings get dirty, the action begins to get jerky and more difficult to do very small and fine little nudges. If you haven't treated the bearings with a little cleaning and TLC, then you are in for a surprise once you do!
Where you set up is also important. Lush turf is the worst possible situation for most dobs. The bottom of the ground board sits on the soft turf itself, not the hard ground, and it means that the turf in now part of the mount, and its sponginess needs to be overcome before the scope begins to move, so more force is needed, and when you release the scope it bounces back as the spongy turf springs back - backlash. Lush turf is not a good surface for most dobs.
Alex.