My own personal thoughts from someone who has lugged his share of photogear thru all sorts of sippery sloggy mosquito infested trails in The Everglades..... I'd put my dollars into a body that can focus quick as lightning. Your lens is up to it, maybe not the very best of the focus speed, but not a slagger either. For sports, nature, especially birds, focus speed gives you more keepers than a lens with 2% better IQ.
There have been times, photographing limkins, scarlet ibis... that tend to stick to the shadows, and come out late and early when the light is not the best... that I could have used IS. Then again, the last time I slogged thru the 'glades I had film and digital that went to 1600 max, and up to 800 was "ok" with noise. Better body, higher ISO with acceptable noise... that IQ difference is seeming smaller and smaller. What makes the keepers? IN FOCUS makes keepers, and catchlights... get a fast focusing body, get a flash and beam extender Fresnel on the front...for those all important catchlights... if nature is your thing.