Its is an intermediate good step but not enough: users are concerned by the fact that adding items to a table is too costly, causes too many calls to the allocator and stresses the garbage collector.
I'm quite sure we can improve this by changing the implementation of tables, adding some controls on how they grow, and by using a sightly better internal representation, and possibly by adding a few APIs to existing tables (but I think that most goals could be done directly in the implementation of tables themselves, because tables are criticial objects in Lua (and almost all is built on them)