just bought my stang

The only 2 parts of the exhaust that you need to do at the same time are headers and mid pipe. But that's only if you ever want to go to long tube headers (or Bassini mid length headers). Otherwise you can do every component of the exhaust piecemeal.
 
No cats+ a decent catback should give you and honest 15-20hp...going from 4 restrictive cats to none really helps.

...this is provided that your car is in a good state of tune, which would require you following the tune up instructions posted above. ;)