Exhaust noise- The amount of insulation if any makes a difference.

The twin outlet oval style is reputed to be quieter than the round single outlet canon styles.
I like the HiTech brand for low noise combined with low restriction.
Yes putting in a hot dog resonator before the main muffler will reduce noise but not by much. (my guess is 2db).

Do you need to retune after changing an exhaust?
Opinions differ, I think- yes (fwiiw).

There are good opinions on here about the Varex but I am fairly sure you need a retune and preferably with a stand alone chip to take proper advantage of it with safe mapping. With Varex, the way I read it, at any speed you can control the outlet to get full open to nearly closed so you can adjust noise to your preference at any speed. Build it in the chip or have a hand control. This is ideal for open road/main road/track when you want power regardless of noise, and want quiet when you are in quiet residential streets. The hand control method may suit your long distance driving.