Before we discuss how fast the lenses needs to be, we need to discuss what you want to do?
My kids are to fast for a slow 2.8 lens! My wife is joking about my "mans handbag" but she likes the pics from my 50mm 1.4 and 85mm 1.4 Sigmas on my old 40D. The 5d M3 is preordered the crop is even not blury enough and I want FF just for that. If you shoot dinosaurs in the museum than f=8 is enough!You will get the bones in the back razor sharp.