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Peter, focal length increases are only as good as the Seeing conditions in your location permit. In theory there is little difference between using a 20mm EP with a 2x Barlow, verses a 10mm EP. However, quality is always going to come into it. There are alot of budget EP and Barlow, and they don't always work well together, in my experience. In your example, I would prefer a good 10mm EP over a barlowed 20mm.
As far as apparent brightness at different focal lengths, this doesn't really make much difference for planets, but yes for dark distributed targets like nebula it certsinly does.
Now back to Seeing, unless the conditions are perfect your going to have a heck of a time trying to force f25 out of your f5 Dob. If you look at a target with say a 15mm EP and see it wobbling around, or looking murky, or stars dancing and spiking, then there is little chance of using high magnification with good detail. There are many folks that carry around very nice 3 to 5mm EP s but rarely get a chance to use them. With experience one can just look at the stars and get a general view of how the Seeing conditions are going to be that night.
Hope that helps.
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