1) Your understanding of taxonomy is outdated by a few hundred years. Linneaus' original system was based on flower morphology, but we since realized that the most effective taxonomy takes into account many different traits. In today's times, we mainly use DNA sequencing to determine relationships between organisms.
2) The leaf structure is not necessarily only optimized for maximum photosynthesis rate. For instance, the leaves of cacti are spines: thin and pointy to minimize water loss and defend the plant. In fact in cacti the stem is the photosynthetic part of the plant, not the leaves. More generally, leaves may be thicker or thinner to cope with drought. You can check out the wikipedia page for Specific Leaf Area for more information.
Long story short: different plants grow in different environmental conditions, so the optimal leaf shape for any one plant depends on conditions like drought and shade.
3) There are many advantages that can be conferred by leaf shape. One example is how leaf shape can affect what is known as the boundary layer of the leaf. The boundary layer is thicker in larger, uniformly-shaped leaves and thinner in dissected or compound leaves. The intro on this webpage seems like a good resource
Overall, good question. You have to keep in mind that any trait is going to be the product of many different factors; no one factor can entirely determine a trait's morphology. For leaves, it's many different factors including photosynthetic rate, drought, shade tolerance, defense against predators, etc. that determine the shape of the leaf.