After a lot of research here and elsewhere, I decided to have the SE Stage III Pro Kit installed on the '09 SERG a few weeks back (have over 3k miles on the kit now). 'Really like the added oomph and the way the 259e cams shifted the torque curve to the right, and the increased rpm. 'Just have to take the time to get the bike back to my dealer to have the flat spot around 3k tuned out.
IMO it's the way the bike should have come from the factory (at least now it will keep up with my stock '05 SEEG) , and it probably would have if the MoCo wasn't saddled with emissions.
Mileage has stayed pretty much the same for me at 40 or so, unless really flying
Some people can get bigger numbers by piecing a kit together, but I didn't want the aggravation as I don't have any good engine builders in the vicinity and I wanted to keep my ESP relatively intact.
My Dealer gives me 15% off and the Harley kit was the most cost effective for me.
All things considered, the Harley kit works for me, your results may vary...