OK, I'm a math guy (not a "beancounter" or an MBA) and I did this type of analysis at AA. Basically, given the Elite upgrade model at DL, the analysis of number of F seats on the 319 is pretty straightforward. Here it is:

4 F seats which are given free of charge to Elite Y passengers who paid a Y fare. (In this case, I make the Y fare minus F food and beverage costs)

versus

6 Y seats which are sold at Y fare (since load factors are at historically high levels, there will be a large percentage of flights where I actually sell the additional 2 seats) and the bonus is that I only have the cost of Y food and beverage (where I potentially can sell food and beverages)

Given that DL will still have the industry standard 12 F seats on the A319, I believe that they probably come out better with a 12 F cabin. That being said, 4 Elites per flight will be disappointed with the new configuration.