I'm pretty sure a free flowing exhaust's main benefits is not the peak power increase, but rather the boost in midrange. Most people do exhaust first because it's a relatively cheap cost to power increase boost.
Front pipes aren't really important, cat back is very common, but I'd rather the whole lot
Most common upgrade sequence I've seen is:
Cat back exhaust, airfilter(pods or high flow panel), turbos/turbos + ecu depending on your turbo choices, ecu if not done yet, injectors, cams, dumps/fronts
Thats just the bolt ons...do internals when they go bang or any point between!