Demur, I think you're on the right track here.  It will nice to be rid of the HAVE_ARGUMENT tests and to not have to decode the arguments one-byte at a time.  Overall, the patch looks good (although it includes several small unrelated changes).  Besides the speed benefit, the code looks cleaner than before.

I was surprised to see that the peephole optimizer grew larger, but the handling of extended arguments is likely worth it even though it adds several new wordy chunks of code.

When it comes to benchmarks, expect a certain amount of noise (especially from those that use I/O or that exercise the C-API more than the pure python bytecode).