Used the Bump Method
I had my first experience with a really "stuck" impeller on my Yanmar 440. It just beat me up, and I almost resolved to removing the entire water pump to work on it on a bench. I do use the impeller pulling tool from Bayshore Marine (pictured from LarryM). I've had this tool for years and if you do not have the the room for the Jasco, it works like a charm.
I went through some suggestions on this thread and tried a combination of ideas. After about an hour of no movement at all, I sprayed WD-40 into the shaft area and let it sit for the night. The next morning I sprayed some more WD-40 and tried again to remove it, with no success. So I then went to the suggestion of something I've never heard of - "Bump the engine". On first try, there was no movement, but after a few rounds of "bumping" and prying with the Bayshore Marine tool (using a board for a fulcrum), I could see some movement (like a 1/4"). Finally, it gave way and slid out.
Thanks for all of the suggestions!