500mm at 100+ feet how sharp?

I have been shooting with my D300 for 3 months now. I have a Tamron 200-500 that I love for it's weight (and price) I always question how good it is. Under 100 feet it is brilliant at times, and to me is more than acceptably sharp. What I often wonder is over 100 ft, it looses some of it luster, over 200 or so, it's just ok. So my question is about the big boy things I might be able to afford if I sell my wife or car and walk 6 miles to work, or just try to save for a year or so. Thoughts would be a 300 F2.8 the older version with a 1.4 or 1.7 goes for around 2 grand or the 300 F4 with 1.4 TC around 1500. Or even an older Sigma 500 F4.5.

Am I expecting too much of any lens (might very well be the case). if anyone has samples of medium-ish birds at 150 feet or so cropped I would be curious to see them.

This Osprey was about 60 feet and I'm not sure how much sharper I could get. The Tern is at 120 feet. Curious what your thoughts are?