exhaust and decal kit.

ride: 2002 Civic Ex Coupe.

If i put a Greddy or some other catback exhaust system on my civic, wouldnt the tuning be off since a change in back pressure is a pretty accurate result. how would it run different and would it need retuning to run te best.

modernimage.net has some good lookin decal kits, but i have never seen them up close. do they look any good, a paint job would cost alot more and if it looks good with decal kit, ide rather save my money for now.