I've had my Magnaflow exhaust in for about 2 weeks now. My observations:
1) My 0-60 times have actually dropped by 0.2 seconds :mad
2) Sounds great. Really makes the SSR sound powerful
3) Gas mileage has improved by almost 1mpg

(I guess this was unexpected)
I will re-examine everything again in 2 more weeks and give an updated report.
ADDED:
The in cab resonance that I didn't like at 1200-1400rpm is now gone, but there is a slightly muted resonance that takes place at 2100-2300rpm which is around 70-75mph.
When not pressing on the gas the exhaust is extremely quiet, but when you get on it, the sound is great.