Synthetics offer extra insurance in extreme applications, like racing, mainly because of their extra resistance to heat and breakdown. I've been using Amsoil Supershift for years and my trans rebuilder said to keep using whatever I've been using.

Some fluids for street applications provide softer shifts for "comfort" and trans life in that particular application and certain "qualities" are necessary for proper computer control. Type F has been used in place of Dextron in race 727's for years because it shifts hard and holds well. Of course, it helps to use aftermarket bands and clutches also. Amsoil Supershift provides the best of everything as far as I'm concerned.