Ok, yield is a totally subjective thing, and it varies so much by strain that there aren't any hard and fast guidelines. General rule of thumb is that short and bushy=Indica/Indica dominant, which also usually means a smaller harvest per plant, but it also means that they flower faster and you get to harvest more often. Sativas (the tall ones) also tend to be harder to grow then Indicas, and take up more space.
As for the lower branches not getting light, it sort of depends. Light passes through the leaves in spectrums that we can't see, so you can't trust what your seeing yourself. Get a lumen meter and see if your getting 5K+ down towards the bottom of the plants-if not, you might want do do a little trimming, but don't go overboard or start taking entire branches.