A slip on end can will change the sound - it may be louder it may change the tone it will depend on shape / size it will be down to your taste as to if this is an improvement or not, check on you tube etc for vids of different exhausts sounds that may help your decision.
The spoilers may fit - as far as the size between the various models (Blob, Round, Hawk) the back end is the same however the mounting holes may differ.
Take a tape measure with you when you go to the breakers and check before purchase. You can take yours off and get the mounting hole diameter and location prior to a trip to the yard.
Ultimately most body kits spoilers etc can be made to fit if you have the time / skillset / inclination to drill holes, fill gaps and blend to fit