Ah, now i think i understand your problem.
Actually, its normal that predatory swiftness gets overwritten by itself. It can (!) happen, if you use a finisher with 1-3 combo points. In most cases this applies to Savage Roar.
The PS proc will always be consumed if you have 4+ combo points or if the buff will expire within the next gcd.
It just makes no sense to use the predatory swiftness proc, because in most cases it will result in a buffed shred - and our highest priority are the finisher.
Nevertheless: i will try to play around with simc - maybe i am totally wrong and i can find another (better) solution.