M1 Max for example has ~240 GB/s of memory bandwidth. Do you think this is because they're using 256-bit LPDDR5@6400MT/s part in combination with not sure how many (?) memory channels or because they soldered it to the chip?

M1 Max has 400GB/s.

Now go find us the benchmark that shows M1 Max is able to saturate the bus with more GB/s than what I said above.

hmm that's a hard one - let's ask Deepseek!