Greg, everybody would say that the Mod GDC is overcropped, except the Marquette grapes (and St. Pepin and others). These are hybrid grapes and they don't know they should follow human and vinifera rules. They want to grow and multiply, so they do.
Interestingly the chemistry is similar across all the training systems. The TWC system actually has about the highest brix, followed by the VSP, them Mod GDC and then the Scott Henry. The Mod GDC is best for the TA and Scott Henry the worst. Mod GDC requires the least amount of labor, then the TWC, then Scott Henry and the most labor the VSP. So you work the most to get the least amount of grapes with very similar grape chemistry. Hmmmmmmmmm. Sound like that is THE way to go(VSP)? Not me.
Over wintering problems with the Marquette on GDC? We have noticed here that Marquette is like St. Pepin. You always get at least a few buds closest to the cordons remain intact over the winter. Those become the spurs, so you get a properly pruned vine when done. This spring when we pruned, we got the highest amount of pruned one year old wood from the Mod GDC indicating that the vine produced plenty of wood for growth and ripening a crop.
As a bit of explanation for how it can ripen a crop about as well as VSP, it has a higher total amount of shoots on the vine than the TWC or VSP because of the second fruiting wire. This allows a higher number of clusters per vine (2 clusters per shoot) which keeps the shoot length in check- around 4-5 feet. Since they don't get hedged, you retain a full set of leaves on that shoot which creates sugars and ripens the crop. With TWC and especially VSP, the shoots become too long and must be skirted or hedged. That reduces the leaves per shoot and lessens the amount of sugars produced. As far as acids, it has been suggested that grape vines produce X amount of acid per vine, therefore by increasing the amount of grapes you are reducing the amount of total acid per cluster. The results so far seem to bear this out.
Remember humans once believed that the solar system revolved around the Earth and that Earth was flat. These are not the results I was expecting when I planted the trial vineyard, but I am keeping an open mind about the results. More studies need to be done. Watch for more work being done with the Northern Grapes Project as part of a multi-million dollar multi-state grant done at the universities.