Upload & Sell: On
You would be asking a lot from any lens at your shooting distance. There is no question that an L lens would increase detail and contrast, but... and here's the key, the combination of shooting distance, misfocus, and underexposure is not giving your lens a "fair-opportunity" to resolve detail. I use 300 f2.8IS w/ & w/out converters as well as a 100-400L for my wildlife work. Both are outstanding, but if I were to be working these subjects at this distance, I'd be shooting with a strong (gitzo or manfrotto) tripod and ensuring that I nailed my focus. You also need to learn how to approach your subject to make the most of your opportunity.
Regarding a lens... If you want the best for the least... 300mm f4 IS... It's old w/ old and loud IS, but it takes converters well, 400 f5.6 great lens-no IS-poor close focus, 100-400L IS - great all around wildlife lens w/ old IS and great image quality untill 400mm (where it softens a tad), new 70-300L IS... Great image quality & af but at 300mm the lens is f5.6... I find this too slow for a high-end 300mm lens. Final option... No IS,, but great older L-lens... oiginal 300mm f4.0 is super sharp and can be purchased for about $700, but beware, if it fails, it can be tough to repair due to the lack of available parts.
When thinking about a new lens, please note that improved technique will have the greatest impact on your images, w/out this, a new lens will only be a new lens.
If you are interested in technique, check out my blog as I'm in the process of authoring 101 tips for the nature photographer (just posted Tip #31): http://bruceleventhal.blogspot.com
Cheers & good shooting,