The engine sound and performance are influenced by what sections of the exhaust? The CAT-BACK or the DOWNPIPES/Sport Cats?

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.