About a year ago, I switched from 45 ACP to 9mm for my carry guns. And I standardized on 124 grain HST standard pressure ammo.
When I read the OP's question, I realized that I hadn't tested velocity nor expansion of this ammo in my guns. So, I headed to my backyard today and did a little shooting.
I shot the HSTs over my chronograph and into plastic jugs filled with water to test expansion. Test guns were an SBR'd silenced CZ Scorpion EVO S3 (7 3/4" barrel)--the closest thing I have to a PCC, an XDm competition (5 1/4" barrel), and a Shield EZ (3.68" barrel). (Unfortunately, I don't have a 9mm pcc with a 16" barrel. I hope the OP will forgive me)
Here are the results:
Average velocity (STDev, number of shots)
EVO S3: 1234 fps (6.5 fps, n=7)
XDm: 1168 fps (11.4 fps, n=9)
EZ9: 1109 fps (7.8, n=9)
And this is how the bullets looked after being fired into water jugs (from left to right EVO S3, XDm and EZ9):
The bullet fired from the EVO S3 expanded to 0.783" (max diameter),
The bullet fired from the XDm expanded to 0.715",
And the bullet fired from the EZ9 expanded to 0.638".
Expansion through all three guns was significant and uniform (although, of course, this is not necessarily indicative of how it will perform in flesh after passing through fabric). The higher the velocity, the larger the recovered diameter. And even at 1234 fps, the HST bullet did not break apart.
The OP asked about best barrel length for the highest velocity from a 9mm PCC using factory ammo. My results don't really answer his question. I don't know that anyone could come up with data that would. But they do show that between 1109 and 1234 fps, HST bullet performance appears to be excellent. Certainly more than adequate for my purposes in my guns.