This will reveal my ignorance, but how does one cool a 400W processor? I'm thinking that some sort of massive copper block with large volumes of refrigerated water might just do it. I can't imagine any sort of air cooling doing the job.
Well, if you can cool a 2-slot 275 W GPU with a single blower, then attaching a taller heatsink via a vapor chamber might work.
I've long thought it'd be cool if they used a vapor chamber as the actual heatspreader, integrated into the packaging.
The other thing to consider is that water cooling is becoming more common, in high-performance servers. And consider that water cooling a GPU dissipating 400 W is nothing new or particularly hard.