for wildlife I'm usually in spot probably like 80% of the time, but occasionally center weighted (I'm more in center weighted when shooting my automotive stuff though), it kindof depends on the situation. IIRC I've got centerweighted at 12mm. I tried matrix on my past bodies and never was really happy with it, but everyone has preferences, if it works for someone, than who's to argue with results. and I shoot a little differently than I did back then, so maybe it's worth another try.
that is a good 6400ISO image too. I can use 6400 on the 800 if I don't crop, the downsizing of the image to web size (or even moderate to small print size) helps combat the noise. but if it's cropped much than I'm just not happy with it at that level, its borderline the same at 3200 too. my normal max limit I put on is 2500ISO.