After doing further comparisons of the Valandre La Fayette with other bags and reading Marcel's comments, I'd like to add the following:
1) I was able to tighten side zip bags (including the Valandre Mirage) effectively under the arm pits and layer with a parka nearly as well as with the La Fayette, minimizing this as a distinct advantage in my opinion.
2) I did, however, see a distinct advantage to the La Fayette when wearing the hood. I was able to sit up in the bag, leave on the hood, unzip the front zip, and slide my hands out for taking care of tasks in the tent without removing the bag (or the hood). This is a real advantage of the center zip design and allows the user to stay much warmer when working in a tent than sliding the bag down.
If you decide to melt snow or cook while using this method, be careful not to get the bag wet! The Microlight fabric (tested) is not very water resistant. However, the fabric used in the newer version will shed water much better and will thus survive a spill more easily.
Thanks for the comments Marcel- and pointing out this advantage that I had overlooked.