Borla doesn't have the low noise and emissions problems to worry about when they design an exhaust like Ford does, so my guess is that the design is pretty much as optimized as it can get.
Another way to think about it is that putting a resonator in there cost them more money that it would have to just leave it as a straight pipe, so it wouldn't be there if it didn't need to be. They could sell a better exhaust cheaper if removing it made for any improvement.
