The extra distance you'll get will come with a loss of quality, maybe small, but it will be there. How small depends on the quality of the teleconverter and a good one will cost as much as a new camera (the one for the FZ-20 costs as much as the camera itself). Other factors such as atmospheric distortions mean that going much above what you already have is often pointless as it just magnifies these distortions - remember the more air you look through (i.e. the farther away you are) the lower the quality of the picture.
You would be better advised saving the money for a better quality camera (may I recommend the FZ-20
? If you really want to go for longer range without compromising quality, you will ideally need a good quality lens and camera combo, such as offered by DSLRs and their more expensive lenses rather than the Fuji (or other point-and-shoots).
Anyone have experience with teleconverters on Fuji's and Panasonics?