eTorque is not going to give you +5.5 mpg, it’s more like <1mpg
You can attribute the improved fuel economy from the last non-MDS 5.7 back in 2005 to the vastly improved transmission (8HP75 vs 545RFE) and the twice redesigned 5.7 Hemi (‘06 and again in ‘09).
Per the EPA, it's 2 MPG bump in the city so I'm not sure where you're pulling that number from... From FCA's own press releases, and it's repeated in 3rd party articles, the 'ol port injected Hemi uses
1.7 ounces of fuel during an average 90 second stop. Since port injection is pretty limited on injector duty cycle, I wouldn't be surprised if 8 cylinders displacing 5.7L uses pretty much the same amount of fuel in any generation while you're sitting at idle. On my 5 mile drive to work, I go through about 10 light cycles in rush hour when lights are backed up, so that's over a tenth of a gallon in just one direction, or ~0.25 gallons in one day.
Let's say you normally drive in a stop and go commute like me and you get 12mpg, that means you're burning 0.83 gallons a day. What if you didn't burn those 0.25 gallons and instead only used 0.58 during that day? Well, that means you'd be getting about 17.2 mpg. Sooo yeah, the more you drive efficiently, let eTorque brake and charge up rather than speeding up to the red light, etc. the more it's going to matter if you sit and idle a lot.
Naturally if you don't have a stop and go commute it matters much less, but for the guy I was replying to it might actually help quite a bit.