I also have a divorced duals setup and I'm very happy with it.
I opted to go with 2.25" piping to match what the Ford engineers chose from the cats on out -- more efficient exhaust velocity but not quite as deep a tone. I initially had 2.5" for the duals before switching to the 2.25" size, and with an auto it made a difference with low rpm torque.
2009 Vista Blue V6 Premium w/Pony Package
X-Charger #276 w/intercooler
Auto, SVT 8.8 w/3.73 T-Lok, Custom Duals, Roush rear springs, GT500KR spoiler, and some exterior and underhood styling.