You don't need to remap it for just an catback akra system. If you're also changing the downpipes, then yes a remap is necessary. Exhaust fitting isn't a particular big job.
Most independent warranties from Litchfield, Autotorque, ACS etc cover mods too, but always ask and check the small print, some require a pre-inspection to be carried out before offering a warranty.
Alot of dealers will try and wriggle out of most claims (or make it very difficult to put in a claim), dash lights, mfd failures, bellhousing, mirror rust etc