If your engine is pretty new, you will need to be patient, else you should contact an exhaust manufacturer that focuses on tailor made applications.
For a CAT-BACK, it has little influence so no real importance. However, for DOWNPIPES/Sport Cats, it is preferable to have them changed before the remapping so this can be taken into account for an improved engine mapping optimization.
This only concerns countries where these technical inspections exist of course. Generally, SUPERSPRINT supplies homologation certificates that are accepted at the technical inspection centers providing that cats are not changed and that central mufflers are used (so use of resonated versions). If we really want to improve sound and performance, this is not enough (as you should use a non resonated version and downpipes + sport cats). Unfortunately, no supplier will supply homologation certificates for these mods and therefore it will not pass technical inspections. In France, the inspections are not as strict as in Belgium and they will be accepted providing cats (even sport cats) are used.
Both influence the sound as well as the performance but not in the same proportion. In general, the DOWNPIPES/Sport Cats will generate 80% of the performance improvement and only 20% of the sound increase while the CAT-BACK will generate 20% of the performance improvement but as much as 80% of the sound increase.
These correspond to the connections starting right after the exhaust headers or if the engine has a turbo right after the turbo and will very often include the Cats.
A CAT-BACK system is the exhaust part that starts right after the Cats up to the end tips. It is called RESONATED if it has center mufflers and NON-RESONATED if these are replaced by tubes.
A Catalytic Converter (CAT) is made of many little cells where are deposited high value metals which will take care of oxidizing the gazes that were not fully oxidized in the engine. A 400 CELL Cat means there are 400 of these cells per square inch. As this number goes down, the amount of physical cells decreases therefore generating fewer constraints for the exhaust flows. The good compromise is often to use 200 CELL cats (typically 400cell is what is used by car manufacturers). If we go too low in CELL value, the oxidation process is not well performed. An engine check light will pop up and it is likely that if there is not enough back pressure you will have the same issues described in answer No2.
For many engines, It is highly discouraged to replace the cats by tubes. Engines require a certain amount of back pressure to operate smoothly. Without this, we can actually see the opposite results and have a loss of power and torque .It is far more interesting to replace them by high flow cats like 200 CELL versions.
Yes. The original parts are not only replaced by improved quality stainless steel tubes, but create less constraints to the exhaust flow which translates to power and torque improvements.