It wasn't really a serious proposition, it was more just satisfying a curiosity.
Hmm I wouldn't have said that an ARM core (if clocked to 1GHz) would be 120x slower than a Xeon core, though I haven't checked any benchmarks and you could quite possibly be correct. I also agree that a four socket 4U server is quite modest, but we're talking about price here. I would have said a modern 4 socket Xeon board alone would cost a thousand or so, and the four Xeon chips would be almost a thousand each, which prices it quite a lot more than filling a box with Rpis
For GPGPU an ARM chip would be in a whole different ballgame, you can't compete with a dedicated GPU with a stream count of several hundred.
Anywho, it will be interesting to see where ARM is in a couple of years.