In a focal-length limited situation, would I get better IQ out of a 400/5.6L on a bare 7D (640mm/5.6) or the 1D3 + 1.4TC (728mm/8)? Put another way, would the extra focal length from the 1D3+TC make up for the difference in pixel density from the 7D when cropping is likely required? I am aware that the 1D3 will only have the central AF point usable in this situation. Will this affect AI-servo performance for BIF significantly enough to minimize any potential gain?
Although this particular setup is for birding, I also shoot gym & field sports. I'm trying to figure out whether it would be worth it to sell the 7D before a rumored replacement actually hits the shelves. Prices for both bodies are only a few hundred away from each other, and I will not be able to afford a 7D2. My current 7D isn't necessarily lacking for me in any obvious way, but I feel that the 1D3 prices are likely as low as they will go, whereas the 7D prices may have farther to fall in light of rumored replacements (70D/7D2). Or would making this switch be a "downgrade" in any relevant departments (video/flash control being irrelevant)?