I ordered a polarizer and a better beamer for the 300 today. The polarizer is a 52mm drop in. Instead of screwing it on the front (which would require an enormous filter), you put it in a slot near the back of the lens.
The better beamer is a flash attachment. It sticks a Fresnel lens in front of the flash. The lens focuses the light. My flash can zoom to cover an area equivalent to the range of a 24mm to 105mm lens. When taking pictures at 300mm or 600mm (using the 2x), most of the light falls outside the picture, so the effective power is much lower. The BB concentrates the light where it will be more useful.
So if any of you are thinking about getting a jumbo lens (like the 300mm f/2.8, 400mm f4 or f/2.8, the 500mm f4, 600mm f4. or the 800mm f/5.6), you need to realize that the astronomical price of the lens isn't the whole story. You'll need a hefty tripod. The most common tripods for the big lenses are the Gitzo 3 and 5 series, which start at $700 and go up. You'll need a Wimberly head ($600) or a Wimberly sidekick ($250) and a really sturdy ball head ($400). You'll need a special drop-in polarizer ($150) if you want a polarizer. You'll need a better beamer ($40). You may also want to protect the finish or camoflage your lens with a lenscoat ($80). Something to keep in mind.