"cunning as a sh*thouse rat"
By that do you mean - not very?
No, I mean "exceedingly". I'm surprised you've never heard that simile (not metaphor) before, it's quite common where I come from, which happens to be where Nathan Greenham comes from as well.
Sorry - we don't talk like that in K-Town South (massiv). We would probably say "mentally agile like a toilet rodent" (in between our prawn sandwiches and asparagus rolls).