I really should know better than to believe most of what comes out of a car salesman's mouth, but after I went for a test drive in an Ecoboost with the 20" Foundry wheels on it (OMG it beat me to death) I asked if there was any difference between the Ecoboost suspension and the V6 suspension. My question was met with an immediate "no". I didn't want my V6's smooth ride (17" wheels stock) to completely disappear, so I opted for 19" with 40-series tires. I can barely feel a difference. Wouldn't this suggest that the V6 suspension is softer?
O.P., if you'd like some 17" wheels with some very smooth-riding 55-series tires, mine are up for sale. :-)