Looking through the 300 mm f4
I don't have the A99 but after the 7D I have or had an A100, A700, A900, A77, and NEX 5&7. The 300mm focused about as slowly on the A100 as on the 7D but all of the rest got noticeably quicker. It was the first 'long' lens that I ever owned and it is still one of my favorites because of its quality and 'relatively' light weight. It has never failed me. You ask how it might look through the A99. I don't use it much with my A900 but it should do just fine. Without the crop factor the image is smaller but put the TC on it and you're back to the original size but with 4 times as many pixels.
Here are a few of my 300mm/7D pictures
The pictures remain while the memories of how hard it was to get them have faded.