Also, does LuaJIT differ any from the stock lua interpreter when it comes to GC performance? It's a continual source of pain for us to have to manually tweak GC parameters on a per-app basis depending upon how heavily lua is used.
Yes, LuaJIT uses its own memory allocator, which improves performance for workloads with lots of allocations. And LuaJIT's memory footprint for most object types is lower, too.
However LuaJIT 2.0 still has more or less the same GC engine as Lua 5.1. LuaJIT 2.1 will feature a new garbage collector, see the LuaJIT roadmap 2011: http://lua-users.org/lists/lua-l/2011-01/msg01238.html
In the meantime, your best bet is to avoid temporary allocations or to use manual memory management with the LuaJIT FFI, see: http://luajit.org/ext_ffi.html
LuaJIT is a drop-in replacement for Lua. Try it with your apps and report back if you encounter any problems. You need to use git HEAD to get the JIT compiler for ARM (disabled for iOS -- don't complain to me).