Just don't expect miracles. Data decompressed on GPU required by CPU still moves back and forth across the PCI bus. before it's ready for use.
I do wonder how things will work out in the future. If it's true that the Navi 2 can give CPU direct access to all GPU memory rather than just a small portion of GPU memory, this may alleviate things somewhat. Instead of going from GPU -> Main memory -> CPU, it can potentially go from GPU -> CPU -> main memory. I believe this is what AMD is leveraging for smart access memory which can provide greater than 10% increase in GPU performance when coded for, even when no specifically coded for, it still provides a small uplift. Of course, the caveat is that it require some level of hardware support.
Combined with DirectStorage potentially allowing the CPU to be bypassed when the GPU loads data from storage, this could get things pretty close.
And then there's progress on increasing PCIE speeds. While it still won't be comparable to the speeds achievable on consoles, PCIE 5 should land in consumer hardware this console gen. And while it was 6 years after PCIE 4.0 (2011) that work started on PCIE 5.0 (2017), PCIE 6.0 (2019) started preliminary development only 2 years after PCIE 4.0 started preliminary development. It's possible that even PCIE 6.0 might land towards the latter half of this generation providing a quadrupling of PCIE speeds over PCIE 4.0.
At ~252 GB/s potentially with PCIE 6.0 with an x16 PCIE slot, the CPU should be able to grab anything out of GPU memory faster than it actually needs it. Heck, that's more bandwidth than the XBS-S has.
At that point, if latency weren't a concern you could render out of main memory (if non-graphics DDR ever gets that fast) with a GPU.
Regardless, it'll be interesting to see how it plays out, and even if it doesn't reach the same speeds as consoles, it's still going to be immensely faster than what we currently have. Just the change in the storage controller software stack and how data is accessed by games is going to lead to a massive performance uplift in loading speeds.
Regards,
SB