You've got that backwards, I think. Putting memory on package, or even just soldered on the PCB nearby rather than in removable DIMMs primarily helps hit higher frequencies or lower power at similar frequencies (GPUs and smartphones provide a wealth of examples). There's little or no impact on latency, because most DRAM latency occurs within the DRAM dies or inside the CPU's cache hierarchy rather than on the link between the CPU and DRAM.
The latency does seem improved in the last level cache and main memory, at least compared to AMD's Ryzen 5000 series. Sadly looks like Anandtech has lost the people who did the deep dives and I can't find similar latency numbers for cache and ram for the Ryzen 7000 series.
Apple's power efficiency allows for a 512 bit wide memory system in a laptop, 4 times any x86-64 laptop. I do hope that AMD or Intel ups their game and allows for more aggressive memory systems on laptops. Only similar design I know of is the Xbox series X and PS5, which both use AMD chips with improved memory systems to keep from bottlenecking the iGPU.
The 16" MBP, at least mine, thermally throttles at the drop of a hat. Teams video conf, backups, virus scan, even small builds, etc.
The M1 in comparison is quiet, cool, not sure I've heard the fan going. So sure the performance is similar, the perf/watt is not.
I don't know if Apple has released any stats, but it makes a certain amount of sense. The technical reason to put memory and the CPU/GPU on a single package is to decrease memory latency (and possibly boost operating frequency). There's probably also the option to have a really wide memory bus.
Of course, a nice side benefit for Apple is that people can no longer buy 3rd part ram at market rates.
That being said...
> My M1 MacBook is significantly "faster" than my previous Intel 16", even though the per-core performance are roughly similar in CPU benchmarks (small advantage to M1).
The thing about benchmarks is, they're sometimes particular to a specific application, and even data sets. So M1 can be largely tied with an Intel chip across a broad range of benchmarks (e.g. Geekbench), but it can also be a lot faster on your specific workloads.