It's not going to scale linearly though. The Max has 400mb of memory bandwidth. If you attempt to use as much as possible, as single Max chip uses 220-240mb/sec. The remainder is likely because they planned to use multiple chips in later products. But 2 Maxs will completely saturate it... so that's going to be a bottleneck if they try 4 Maxs. And there would be little point going above 4.

For comparison, the 3090 has almost 1TB of memory bandwidth.

Not a dumb question at all. You can to an extent, but the returns diminish quickly. It isn't linear.

This fact however makes "performance per watt" comparisons misleading between different processors designed for different environments (e.g. M1 vs AMD Desktop). It takes a more power to get that much extra perf, conversely and the more under-appreciate part IMO reducing the speed a little can save a ton of power/heat if the chip is currently running at the higher power portion of the curve.

On a desktop machine most people would want them to tune for performance at quite a substantial power efficiency cost so of course a desktop chip most probably is less power efficient per unit of compute. You don't need to power it with a battery after all and there's heaps more cooling capacity in a bigger form factor so why optimise for that?