That's true. I just would not expect a
significant improvement with the 70-300 non-L (Unfortunately the Canon EF 75-300mm f/4-5.6 III USM is not available in the image quality comparison tool). Bryan recommends the 70-200 f/4L as an upgrade
http://www.the-digital-picture.com/R...ns-Review.aspx. Even with an extender, it will be better than the 75-300, so sedwards already had the right idea asking about this lens.
To add to that point: I would not recommend
replacing the current lens with the EF-S 55-250. Instead, I would stick it out until I have the funds available for one of the 70-200 options, maybe with an extender if more focal length is needed.
EF-S 55-250 is great if you don't have any tele zoom yet, and if you are not sure if you want or can spend the money for one of the better options (it was my gateway drug :) )