Although they are a different type of grape, I grow several dozen merlot and cab vines and this is a frequent occurrence for us. We always remove these extra clusters for several reasons. If we let them hang, it would stress the vines as they now would be setting too much fruit. Also, the new clusters would be several weeks behind the primary flush- making the wine process impossible to time. Last, the cooler weather would most likely arrive before the brix/TA reached proper level. Like you, we thin the leaves every year so that the bunches hang in light but we need to shade a little more than other parts of Cali as our sun is more relentless. We do this so that all of the fruit will ripen close to the same time and to lessen the incidence of powdery mildew and bunch rot. Also, it is worth noting that many vintners advocate sun exposure in order to develop the more complex flavors associated with a particular grape or style of wine. Canopy management is such a science within itself.
