Thanks. Premature optimization is the root of all evil said someone much wiser than I. My intent here is not so much to optimize as to avoid code carelessly sprinkled with a multitude of local constants with all the problems that engenders.
It also makes it a whole lot easier to make global alterations to the code. A case in point - acting on @britzl 's suggestion I dropped trapping the collision_response event and switched to trigger_response instead. Given that my comparisons were being done against a pre-defined string in a Lua module the switch involved about 0.5s of work.
Doing such things in the vain pursuit of saving off a few nano seconds here or there is, needless to say, an utterly futile exercise