Need help shooting action in low light

Hi all,

I was at a horse show this weekend (Pat Parelli show if anyone was there) and had some problems shooting in the mixed light. I'm using a d7000 with sigma 50-150 2.8 lens (1st gen). I had the shutter speed set to around 1/250 with either f2.8 or f3.2 (the first gen is a little fuzzy at 150mm & f2.8, it sharpens up at 3.2), ISO was on auto mode with max of 1600 initially, later increased to 3200. I was initially using whole scene exposure metering but later switched to center weighted. I shot in RAW.

Some of the photos came out great, a little underexposed but a stop or so of exposure push in post made it right and there was little noise and good detail. Some at 3200 were very underexposed and pushing them in post just revealed a blotchy, grainy mess For the underexposed shots, if I had shot at ISO 12800 it would have been properly exposed (I'm guessing here given that lowering the shutter speed 2 stops gave me a properly exposed picture, I didn't try it out with raising the ISO).

My question Given that this is action photography I can't go any slower with my shutter speed (and really needed faster in many cases), and I'm shooting my lens wide open. Is it better to push the ISO up even into the 12800 range and get a properly exposed photo. Will this have less noise and more detail than underexposed (at ISO 3200 or less) and raising exposure/shadows in post?

Thanks everyone for your help ...

Berardino