Yes, heat is going to be substantial, but 50W is not too crazy for a chip with a large heatsink. Pictures of testing heatsinks have been posted as Carlton is mentioning.
On the first question - you probably haven't looked at the specs. (...)
As for the heatsink - I'm not talking about what they plan to put in their retail products. The question is specifically to the chip's packaging. The final product design is a consequence of the chip design.
If the chip has exposed silicon - they you'll see heatsinks on top of the chip (e.g. like BFL). If the chip is meant the send all the heat via the PCB - then you'll see a large heatsink on the back of the PCB (e.g. like Avalon and Bitfury).
So - the question is which of the two options have they picked (and if that's been decided already).
You know very well they use FCBGA packaging. You specifically asked them if they could switch to QFN:
After taking a quick look at the specs - I have some questions (and recommendations) for the development team:
Do you HAVE TO use a FCBGA and 700 pins? That's quite a pain for troubleshooting and not to mention much more expensive for in the manufacturing process.
If it isn't too late - could you consider a QFN (or[T]QFP or something else non-BGA)?
Do not ask to switch to QFN packaging just before tapeout and for a 50W chip.
Such a proposition clearly shows you either have quite small knowledge about chips or you try to troll the topic.