I gain a lot of edge with sharp micro; particularly medics and scvs tend to have vastly extended lifespans if you micro 'em a bit, and tank push is about 1000 times more efficient when performed correctly, with scanner support.
Overall, I find you don't need that good micro for Brutal (the computer is still just a computer, after all), but it certainly helps, especially if you try and accomplish all the feats of strength (efficiently fighting Toss with Storm before you get Tanks is very micro intensive, for example).