I know this thread is old, but just in case if anyone wanted the answer..

When the mechanic installs a new exhaust piping (whether its the cat back or the header), if the pipes are of different sizes they put in what they call a 'flange' and this ensures that the two pipes connect directly to each other. Different flanges are used for different set-ups. So in a sense, there shouldn't be any gaps, but your only problem is back pressure

e.g. headers = 450mm --> cat back = 300mm ... then you'll experience what we know as bottle necking (good low end, crap top end)
headers = 300mm --> cat back = 450mm ... then you'll lose most of your back pressure (crap low end, good top end)
essentially the best set up is to match the pipes (e.g. headers & cat back = 450mm).

for the second question, the only thing you'll save on is the flange which is less then $100 (from memory)

hope this helps, and if anyone can correct me.. please do. =)