I have the Force II and have no issues with them at all. I have the stock manifolds. If you are going to do long tubes then I would start there with the stock mufflers. It might be loud enough for you and you can save some money. If not, then get mufflers.
I have a bunch of clips on youtube of the Flowmasters all by themselves too.
FLOWMASTER FORCE II Axle Back Exhaust 2013 MUSTANG V6 - YouTube
2013 mustang V6 Flowmaster forceII - YouTube
Those were with no miles and then 100 miles them.
They got even deeper now that I have about 2,000 miles on them.