Hey, guys. Interesting discussion. I wish I had a couple of hours to try to wade in on this, but unfortunately I have a ton of work to get through this weekend.
As for the stomatopod eye, it does not have the "best vision", but it certainly has the most complex receptors. Gonodactyloids do see across the broadest spectral range of any known animal - 300 - 700 nm and they have up to 16 different visual pigments (rhodopsins). They also that up to 4 colored filters, three different polarization analyzing systems and a range-finding system that effectively gives them hexnocular vision (compared to binocular vision).
It would not be correct to say that all of the visual processing takes place in the three ganglia in the eyestalk, but certainly more does than in our retina. The brain still must integrate information from the two eyes, determine what is relevant and act on it (or not). In arthropods, much of this processing and integration takes place in areas called mushroom bodies - analogous to to our visual cortex. In flying insects, these structures are very large probably to provide rapid and detailed analysis required during flight. They are much smaller in stomatopods. In neither case does this have a lot to due with complex learning
Octopus have relatively large brains - larger than those of stomaotpods. However, about a fifth of the brain is devoted to the control of the chromatophores and skin muscles that allow the animal to change color and texture. Stomatopods don't need to do that. Furthermore, octopus do not have a hard skeleton. Controlling body movements is much more complex in cephalopods because of the need to coordinate different muscles. If you think about it, to move a leg at a joint in an arthropod or a person required only the control of a few muscles. To do the same task in an octopus arm requires the integration of dozens of muscles. (To know where the arm is also requires a lot more sensory input in an octopus.) I could go on with a similar discussion of dolphins. Their sonar system occupies a considerable portion of their brain. The same is true of weakly electric fish, bats, etc.
As for the phylogenetics of stomatopods, they are all in the superorder Hoplocarida which split from other crustaceans over 400 mya. Most biologists now place them in the Subclass Eumalacostraca and in the Class Malacostraca. There are four superorders in the Eumalacostraca: the Hoplocarida, the Syncarida, the Eucarida and the Peracarida. The Eucarida contain many of the crustaceans that people are familiar with. The order Euphausiacea include Krill and the Order Decapoda that contain the shrimp, crabs, and lobsters.
The Class Malacostraca includes the Eumalacostraca as well as the subclass Phyllocarida - a small group of most suspension feeders. A few classifications place the hoplocarids as a sister group to the Phyllocarids, but I think that is pretty much dead as more molecular evidence comes in.
As for the mandibles, I've attached a photo of a pair of mandibles (moltskin) from a Lysiosquillina maculata. The "L' shaped serrated teeth on the two arms would normally oppose one another. They are effective at cutting and sawing apart chunks of prey. I've even had large lysiosquillids and Hemisquilla chew a hole in a heavy nylon mesh dive bag in a few minutes and escape. If you look at the contents of the stomach, the material is usually completely macerated and unidentifiable.
Roy
