There are some significant changes, but most aren't cosmetic. The lights changed in 92 along with the truck, and the corner marker lenses were on some and not others. In 93 the wheels changed to the style that carried through to 95.
As for mechanical, braking systems were different, fuel tanks were different, exhaust design changed with the fuel tank, MAF sizes changed, along with throttle bodies as well. Location of the wiper fluid changed in 93, rads went from brass to plastic in 92, and spindles were upgraded in 93. Camshafts were different for almost every year also, and fuel pumps varied in size.
I'm sure there were many more changes, but those are the ones that stand out in my mind. The cams would be the only factor that would have made one SC "faster" than the other, but it would be margin, if measureable at all.