It's an underwhelming product in an annoying market segment, but 256GB/s really isn't that bad when you look at the competition. 150GB/s from hex channel DDR4, 200GB/s from quad channel DDR5, or around 256GB/s from Nvidia Digits or M Pro (that you can't get in the 128GB range). For context it's about what low-mid range GPUs provide, and 2.5-5x the bandwidth of the 50/100 GB/s memory that most people currently have.

If you're going with a Mac Studio Max you're going to be paying twice the price for twice the memory bandwidth, but the kicker is you'll be getting the same amount of compute as the AMD AI chips have which is going to be comparable to a low-mid range GPU. Even midrange GPUs like the RX 6800 or RTX 3060 are going to have 2x the compute. When the M1 chips first came out people were getting seriously bad prompt processing performance to the point that it was a legitimate consideration to make before purchase, and this was back when local models could barely manage 16k of context. If money wasn't a consideration and you decided to get the best possible Mac Studio Ultra, 800GB/s won't feel like a significant upgrade when it still takes 1 minute to process every 80k of uncached context that you'll absolutely be using on 1m context models.