Man, your post makes me feel like Ubercart is the fat guy in the pool squeezing all the little kids out. I don't have anything helpful to add to the thread atm, but hopefully we can work on these memory troubles w/ D6 and the new menu system. I'm not a great memory optimizer, but if you know of any things routinely used in PHP that eat up resources, I can let you know where we might be biting you and how we can improve.

This won't help for now, but at least when the site goes live you can disable the devel module and hopefully free up some resources. Same for the Views Theme Wizard. Also, does your memory comparison include disabling Workflow-ng?