I have a 2007 Ram 1500 with the 5.7 HEMI and a 3.55 diff. I can get near 20 on the freeway if I stay on flat ground between 55 and 60 mph. Over 65 and it really drops the economy.
The 2006 1500 models were the first to get the MDS system. With a stock exhaust system, the exhaust tone is hardly different when the MDS engages. I have a Flowmaster muffler and twin tailpipes. I can hear a change, but it isnt bad. I would expect it to be more dramatic with a cheaper or louder type of muffler.