Maybe the larger Girodisc rotor gives the brakes slightly better feel over stock. It means there can be a little less pad/brake pressure to achieve the same braking force but maybe the noticeable benefit is more psychological. I think the floating rotor design on the Girodisc (rather than the fixed OEM) can also help with brake feel under some situations.
The Porsche 6 piston calipers also require a larger Master Cylinder so you are looking at unnecesary extra cost IMO for most purposes. I think even the GiroDisc upgrade for most people is not necessary but if you need to replace your rotors anyway then it is a nice upgrade. If you were doing a very long race, maybe 1 hour or more I think the larger 6 piston caliper and rotors become more important, otherwise the GiroDisc seems plenty good enough for many track drivers.
The braking surface on the GiroDisc is the same size as the OEM rotor so the pads do engage the full width of the braking surface. You can see the bedding in on the full width of the rotor surface in the photo below. I think if you compare this to the bedding on the OEM rotor above, maybe the coverage of the GiroDisc is a little better/more precise.
My OEM pads were still in good condition so I kept them but sure a lot of people upgrade pads with the rotors. However, if your pads are still good you can have your workshop burnish the pads and bed them in as per usual. I don't think there is any disadvantage, but as noted above some people say you should not reuse the caliper bolts too often if you are changing pads regularly, which in any case you should also get the caliper stud kit.