Comparing GFlops is meaningless here. FLOPS is FLoating-point Operations Per Second, and Primecoin uses integer arithmetic. So, even if the Phi is so much faster than X86 in floating point maths, it means nothing for Primecoin performance.
Yeah, you're right. This was the best i could come up with to compare integer maths between processors:
http://speedtraq.com/?s=1Unfortunately the Phi isn't on here, but if we choose something that is relatively close to it:
Intel(R) Xeon(R) CPU L7555 @ 1.87GHz, the result is: 88.478x
Where as with the
AMD FX(tm)-8350 Eight-Core Processor, the result is: 29.681x
So even a Xeon that has more cores, and a higher clock speed than the Phi is still only 2.9x faster. Which means all it would take is three builds with those 8350s to outperform the Phi coprocessor. And now i think we are coming in cheaper than the Phi going with AMD, but it really depends on how much the rest of the builds cost (mobo, ram, hdd, psu, etc)