The 3.6 paired with the 8 speed is an awesome combination. When I was working for FCA one of my company cars was a 300S and it was my favorite. Super low gears give great acceleration and got nearly 30mpg on long trips. 3.6 doesnt have the same issues with eating cams or dropping valve seats as the hemi. The oil cooler can leak, which is a pain as its under the intake.
The dropped valve seats and cam eating was resolved on the 5.7L engine when the Eagle variant was introduced in 2008. My 300C with the 5.7L Eagle Hemi and NAG 1 5 speed auto gets close to 30 mpg highway.