I found that Independent Outbreak High Population is not enough for high amounts of PI. I tested it deeply for few days. Some system will give 5 x 3 and nothing after that (checked some of them multiple times in different tactics to "force" respawn). Some will fill your cargo in an hour. There must be additional condition for high drop count (I used all known tricks).
Right. I tried Epsilon Indi before (after read your post) - system was still in Outbreak - and found NONE High E. [blah]
But short time later the state of Epsilon Indi changed to N/V.
So my guess is, if you enter a system where the state Outbreak (or any other you look for - like 'Boom', etc.)
is at it's end, the chance for a High USS is not longer there. On the other hand, if you enter a system
and the system state just changed actual to the state you look for, then may be the chance for High USS are highest...