The 08 SRT catback should fit, however you will need to reduce it from it's 3" to the 2.5" of that on your RT. It should give you a little more power as the SRT are a little more "free flowing" than that of the Rts.
If you ask 20 people about CAIs you'll probably get 20 different answers. I have some different ones on my cars. I have a LegMaker "Hammer Design" on my SRT, have a Big Bully on one of my RTs and a K&N on my other RT.
IMO the Legmaker is the best, however the most expensive. Money wise, I'd recommend the BigBully ~ about $100 less. IF you want the BEST then I'd say the LegMaker!
And from a SIX MAGNUM family in North Carolina.
Mine: 06 Mag SRT, RED 05 R/T & BLUE 05 R/T & a SuperCharger (663RWHP 300SRT)
Wifes' 06 Mag SXT, RED 05 R/T & BLACK 05 R/T & a "BLACKED OUT" 300SRT