As per Monoprice, they do carry what you need here for under $10 (type in "breakout cable"). Cheapest I've ever seen a MiniSAS breakout cable (usually see them for ~$30)
The one you linked is the SFF-8087 breakout cable. I need an SFF-8088 -> 4x eSATA breakout cable (external). That they do not carry and I must purchase elsewhere (might as well be from OWC since I don't get charged tax from them).
Unfortunately, I suspect this issue is with your MP, not the cards themselves.
One thing I can think of (if you have the time and inclination), is the Slot Configuration Utility isn't working properly in newer versions of OS X, so you could try moving back to the original version of OS X, and run that version of the utility. Then see if it works. If so, then upgrade the OS to a newer version, and see if it still works.
Not sure if this will do what you need, but it's something you can try.
Fraid that ain't happening - the current video card I have (and my fallback) both require 10.6.4 + Snow Leopard graphics update minimum for acceleration, and will function in 10.6.0-10.6.4 vanilla, as well as 10.5.8 only, as a 2D unaccelerated raster device with no resolution switching support. Earlier (10.5.7 and below) KPs on boot with these two video cards, a 5870 and 5770 respectively. The original X1900XT has been donated to a family member out of town already.
We'll see if it was my system or if it was the card (Areca) I had before that was the problem. If this card works at x8, then the Areca had either an incompatibility with the 1,1 Mac Pro (that particular card at least) or was defective from the getgo. The only thing I know for sure at this point regarding the Areca card is that it was not one of the initial batches based on two things: It had the new unified heatsink, which beats the everloving crap out of the dual postage stamp heatsinks for dissipating heat, and it didn't make my Mac Pro not boot when installed. The original batches had incorrect capacitance and would cause the MPs to fail to boot at all.
I'm keeping my fingers crossed. $8 shipping, no tax and the discounted price gets the card here on Tuesday. If it works, then I have a nice solution for up to four 6 Gbps SSDs. If not, I send it back and eat a restock fee. Since the price is $279, I eat about $35. I can live with that. Either way, this card will tell me if it was a fluke card (Areca) or my system just isn't gonna take a RAID card and I should use it as a hand me down to my grandparents after getting an SNB Mac Pro later this year/early next year.