Warm tier is live. Workers mmap the VPA files over NFS instead of loading into RAM. Expected a real performance gap. Didn't get one -- while the working set fits in page cache, warm is basically as fast as hot. Queries come back in the same ballpark. I had to double-check the config to make sure I was actually testing what I thought I was testing.
The "warm cliff" -- where page cache pressure starts forcing actual disk reads -- is at way higher collection sizes than I assumed. Good news for the free tier promise. Means the real hot/warm difference is about capacity planning, not speed. Hot gives you guaranteed memory residency. Warm gives you best-effort and the effort turns out to be pretty good until you really push it.