Yes the energy density is the same. But my point remains: it is technically easier to cool 4 x 63W chips than 1 x 250W chip. So I am waiting for Simon's reply...
4 x 63W chips means four fans and four heat sinks versus 1 water cooler and heat sink for the 1 x 250W chip. Some of AMD's server chips run at 80W with mobos having 4 sockets and they all require fans and heatsinks.
Right, and they require cheaper fans and heatsinks as well.
Multicore chips need a huge amount of bandwidth between the cores. Each core can actually read eachother's cache at full cache access speed. They need to do that to implement thread memory locks - as soon as one chip changes a spot in memory, all the other chips need to be able to see it. (Although this is different in some multicore cellphone chips to save energy)
bitcoin miners barely need any bandwidth at all. If you split up the chip you could use water cooling on all of them and potentially pump more power through them and get more hashrate for the same amount of silicon. You would have the same w/mm^2 of silicon, but each one could have more total heatsink area.