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The short pistons are for using 1.25" thick rotors as opposed to the standard 1.1" thick rotors. If you're using stock rotors, you don't need the short pistons.
Another option is stainless steel pistons (instead of the stock aluminum pistons). Stainless doesn't transfer heat as much as aluminum, so it helps insulate the brake fluid from heat generated by the pads. However, for street use, it's not necessary.
As far as price, there are a lot better deals out there than DRM. Don't get me wrong, they are a very good shop and have some very knowledgable people and good prices on some of their stuff. However, for the GS calipers, you should go elsewhere (18carfan on the forum is a good source).
I got my C5 setup from Jeff(18carfan). Costs about $700 for everything and is 10x better than the GS system. Jeff had Z06 calipers for under $100 a piece shipped. I'd get the GS from Jeff or if you are willing to spend the extra few bucks upgrade to C5 brakes.