The reason I asked about the taste is I don't know the growing season in CA. The sugar content goes up the longer the grape is on the vine. Many times grapes are bitter before they are ripe. This indicates there is not enough sugar yet. Some grower go by taste, others test for sugar before picking.
Assuming a 1.060 OG and a 1 FG your alcohol content will be right around 8%. I don't know what is typical for a Cab. You could let it go as it is and then add sugar later. I guess it depends if you are looking to have a good tasting wine, or something to get you drunk. I don't think increasing the ABV just to have higher alcohol is the way to go. Yooper was right about thinning the wine. I don't get much into the reds, but I think you are aiming for something full bodied.
You may want to speak with the vineyard owner about what he does with these grapes.