Does the feat Acrobatic help a bard that's using Perform (Dance) for his acrobatics check? The feat says that it helps with Acrobatics checks, which technically this isn't; it's a Perform (Dance) check. But an armor check penalty would still apply to the acrobatics check, even though it doesn't apply to the perform check.
All the older threads I found ended in house rules for what modifies a versatile performance substitution. Is there an official clarification anywhere?
Nope.
Your assumption is correct. If I'm using Perform (Dance) for Acrobatics, I don't take an armor check penalty and I always use Perform (Dance). If you get, say, haste cast on you, and get a bonus to your jump checks, you gain that bonus to acrobatics-- not Perform (Dance).
So, Perform (Dance) at +12
Acrobatics at +5. We put one rank in at level 1 to cover us until we got to versatile performance.
example 1.
I put on boots of striding and springing. Acrobatics to jump is now +10. Perform (Dance) is still +12.
example 2.
I get an ioun stone of charisma and a circlet of persuasion, getting a +2 to Charisma and a +3 to charisma-based checks. Now my Perform (Dance) is +16, and my Acrobatics is +5.
I'm pretty sure that's exactly how that works.
I found a thread where Mr. Jacobs says he'd let magic items stack their bonuses. I'm not sure why it's so mushy.
I was just asking about this in another thread. Can you provide a link please?