Re: 09 V6
Do NOT remove the cats. The 4.0 sounds terrible without the cats; everyone I know who did it regretted it. The long tubes generally require you to remove the cats, and the performance gain isn't enough to offset the exhaust note. Shorties will give you a little deeper tone, not much performance again.
The flowmasters have a great sound. I had the Flowmaster American Thunder, but took it off because the drone was too much on the highway. Sounded great outside, but was too much on a 45 mile one way commute every day.
If you go for the Supersix heads and cams you will need to find someone to do a tune for them - and to set them up. You need someone who understands how to use a degree wheel for the set up on the 4.0