Sorry that I didn't mention it right away, but what is your PCI-E speed when you're trying to go over 221?
I had best results if keep PCI-E speed half of the BCLK.
Hmm... Strange... As you said, there must be something more. I wouldn't be satisfied with the answer: one PCI-E lane is to blame.
If there was any other way to test...
I think I found one: Buy ASRock SATA3 Card for 25-30$ (sorry for saying so directly: go and spend money). It is PCI-E x1 SATA3 controller with Marvell 88SE9123 / 88SE9120 chipset. Similar chipset, still one lane used, but different aproach.
Maybe you get more conclusive results.
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