does the full exhaust system for a gs500 replace the header pipes? If so, are they at all different? is it even worth getting a full system, or should someone with a gs just get a slip on?
Yes a "full system" replaces all the OEM exhaust pipe. A "slip on" usually replaces the header somewhere past the collector (where the two pipes come together). On a GS this requires cutting the stock pipe, thereby making the change irreversable (unless you find another OEM exhaust).
Most aftermarket full systems have a 2 into 1 config with a collector designed to improve top end power. The stock header with its cross over and stepped collector is designed to even out the midrange. Most of the considerable weight of the GS's stock exhaust system is in the muffler can, so putting a slip on with both help the bike breathe better and loose some poundage. The "problem" is that the price difference you find between a slip on and full system for a GS is relatively minor (compared to I4 sportbikes etc) at a $100 or so. You can often get used V&H or Yosh pipes for the same price of a new slip on...
Is the stock exhaust the quietest of all the avaliable pipes we can fit on our bikes?
I just don't want to install a loud noisy thing. I like my bike quiet.
yes