I shoot quarter horses at a local track. The D4 tracks fast, focuses fast and so far I haven't hit the buffer limit shooting at 14 bit RAW. There will always be a better camera with higher resolution, faster processor, better low light...etc. Right now, you are at the limit of your equipment. Another alternative is the new D300s replacement. It is supposed to be announced in September. If you are looking for a FX Pro body for action, D4 is it.