[mascarenhas]

Lua numbers are not boxed in the usual sense; they live in the Lua data stack (which leaves in the C heap, but this is not isomorphic to the Lua heap), are not accessed through a pointer, and do not put pressure on Lua's memory allocator and garbage collector.

LuaJIT 2's NaN tagging alone does not speed up the stock interpreter: http://article.gmane.org/gmane.comp.lang.lua.general/44823.