But if they were to benchmark and see that things like that pack method were slow, I think it is unlikely they would be able to implement the algorithms needed to improve on this kind of code, given their current implementation techniques.

Rubinius is essentially a template compiler, emitting a chunk of LLVM for each byte code. There isn't any sophisticated optimisation before it goes into LLVM, so nothing to for example partially evaluate a sort routine or remove allocations. The LLVM that comes out is far too complex for LLVM's optimisations to work for them.

JRuby relies on the JVM to do the sophisticated optimisations, and C2 (the server compiler) just doesn't have the optimisations or inlining scope needed to simplify code like the pack example. JRuby are massively improving on this with their IR, but they are going to have reimplement some very complex optimisations themselves to make this work on methods like pack.