It seems to me that several themes have run thought the postings on the "hog" in the past:
1. Tele-extenders reduce the quality of the photo (usually 30% degradation is quoted)
2. That you must use a tripod when using a "long" lens to get sharp photos
3. That shooting from a boat will blur the photo due to engine vibrations
4. Traveling with photography gear requires a hard shell Pelican case, etc.

I would like to produce evidence that none of the above have to be true. Part of me wonders why I bother, because "a man convinced against his will, is of the same opinion still", but I'll still try.

I just returned from South America & the Galapagos Islands which was a very disappointing trip. I did not see what I expected. The Park service controls where you go, so maybe it was just bad luck, but disappointing none the less. Most of the advice that I got for the Galapagos was that long lenses were not needed, as everything was so close. I hoped that was true, but took my 300mm and extenders just in case.

I travel extensively with my gear, and use think tank products which have proved very adequate. Normally I use two of their large roller bags, but this time I "went light" and took one roller bag and a large back pack. I have trucked this stuff to more than 80 countries with no damage. I do not check the bags, but place them overhead as carryons. I am 76 years old, and it is getting tough to lift them up, but I make it, so far.

All shots attached are with the 300mm f2.8 with the version III 2X tele-extender and all are hand held. The hawk shot was from a boat with the engine running, the others were walking. No tripod. Download the attached files and check them out. I think they are pretty darn sharp!

About tele-extenders: There are good ones and bad ones. I understand that many readers have limited budgets and try to save on gear; and I have been there. I am blessed that I now have the gear that I want, and high end tele-extenders are very good. Check out Art Morris's photos at birdsasart.com and see his results with tele-extenders. Don't lump all tele-extenders into the same group.

In stead of saying you can't do this or that; concentrate on learning better technique, learn how the pros do it, and get better with your photography.

Keep shooting!
Bill

There is nothing wrong with compromising quality when it's the only option we have. But we shouldn't fool ourselves​ into believing some add on gadget will be a magic bullet. You took the route you did to get the best results you could within your budget. But you recognized the compromises and the limitations. The OPs lens is one of the sharpest Canon makes, and it's known to work very well with excellent results with both the Canon 1.4x and 2x Extender III. But that should not fool us into assuming that there is no image degradation from that combo.