UNPACK_SEQUENCE already has fast paths for tuples and lists, which make up (literally) 99% of unpackings in the benchmark suite. What's more, two-element tuples make up about two-thirds of all unpackings (though I actually suspect it's even higher, since the unpack_sequence benchmark is definitely skewing the results towards 10-element lists and tuples).

These specializations are trivial to implement and result in a solid 1% improvement overall.