FA carriers are made heavier because more mass is needed to slow the cycling rate of full auto applications.
Strictly speaking, this is incorrect.
The extra mass of a FA carrier is the byproduct of what it is made to do, not the reason it is FA and not a semi. At the bottom rear of the carrier the full auto carrier has the bottom and top notches cut to the same relative spot (on the top and bottom)... that is so that when the bolt and carrier are all the way back in battery the bottom of that carrier catches the sear when in full auto (or burst) mode, causing the sear to let the hammer go again.
What makes a semi carrier a semi carrier is that the carrier is cut in that spot- usually just cut back a bit, sometimes cut all the way (as is the case with some Colts). Then it is unable to trip the sear if one is present. This prevents someone from just adding parts to a semiauto rifle and making it FA (there's some other tweaks to the lower that are often done to prevent this too).
That extra material back there is what increases the mass. It isn't that it has more mass and that makes it full auto, it's made to be a full auto carrier and that's why it has the extra mass.
As for the firing pin shroud, it's not done on every semiauto bolt carrier. Heck, I don't even have one that's unshrouded... not many are nowadays. What that does is catch the notch on the hammer (if the semiauto hammer is notched) to again prevent the gun from running full auto. It's more a precaution to prevent someone bubba'ing an AR into a full auto than anything else, but it's another thing that keeps the maker from getting into trouble from those who accuse them of selling "easily converted" guns.
FA carriers are often preferred, but in actual use any difference is minimal and I wouldn't set about replacing a semi carrier with a FA one if I already had a quality setup in the rifle.