I understand, but 100Mb/s hard to grade 8-bit 4:2:0 Sony IPB footage shouldn't outright beat the 272Mb/s All-I 12 bit 4:4:4 out of the P4K. I expect higher quality for higher file sizes. Plain and simple. If the higher file sizes bring worse quality, then they're not worth the higher storage cost associated with them.
This is not an accurate statement. The Sony gives you h.265 (or h.264) which are highly optimized codecs, and require hardware acceleration to decode. Basically, you do get smaller filesizes, but to decode them, you need to sacrifice a goat. You don't see that, because every GPU today can decode these formats. So for you, you get the impression that it's a "normal", "run of the mill" codec. It's not. It's HIGHLY dependent on hardware acceleration. Without it, you will be getting 1 fps.
BRAW on the other hand, it runs with minimal hardware decoding needs (it does use the GPU, but not for everything). And it gives you 12bit and 4:4:4 (which by themselves they REQUIRE more bits in order to fit in that bitrate).
So yes, the macroblocking you're seeing, is correct. A less hardware-dependent codec at 12:1, with added bonuses of 12bit/raw (which are bit-hungry), will perform LESS well than any modern codec at a lower bitrate, should that codec is highly hardware-dependent.
Basically, you trade size for hardware acceleration. Unfortunately, you can't create a new codec (like BRAW) and still get FULL hardware acceleration on every level like h.264 does, because that requires the GPU chip to be specifically designed for your codec. And nvidia/AMD/Intel don't care about BRAW (they barely care about Prores either). So, what you're seeing, is correct, and to be fully expected.