I think Soft is holding up incredibly well by modern standards from a pure performance perspective. It's biggest speed killer these days is the viewport (although for pure particle effects, it's still pacy). But nothing is a match for the fluidity of workflow in Soft, and as the Arnold team like to preach, artist time is more valuable than processor performance!
ICE kinematics are very fast but it's an unfinished toolset with many quirks and restrictions so it's hard judge it as being fully production capable. That's not to say that you can't take advantage of it's strengths (lean rig setups and raw rig performance), and then Alembic out your assets for scene assembly in the likes Houdini or Clarisse.
Now that Parallel Evaluation in Maya is reaching maturity it's most definitely the most performant character animation DCC toolset. That, coupled with all the smart behind the scenes caching makes a powerful combination. But there are many toolsets that TD's rely on in Maya (such as the SOuP suite of operators) that necessitates Parallel Evaluation to be disabled). Even in this second decade of the new millennium, CG production is all too often throttled to minimal processor core utilisation. And with that it mind, I think the good ship XSI still has a few laps of the circuit in her before her wheels fall off...