Personally I've done between 80 and 100 SR runs (sorry for the approximate number) and only have 2x Perf aug pieces (Ta'lab Trousers and Witching Robe).
Seen at least 3 more perf augs pieces though, which I didn't take (2x perf augs WHM gloves from Ingrid, and another item I didn't want).
I think there's a table with a small number of possibilities. Each time a dice is rolled and according to the number rolled, you get different quality of augments. If the max number is rolled, then you get perf augs.
Per Augs rate can't be lower than 5%, leaning to think it's gotta be more around 10% if not even 15% like Fwahm reported.
Even more because the people I usually do SR with got more perf aug pieces than me, which also leads me to think I've been unlucky and not on average so far.
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