has anyone heard of submerging a plate chiller in a bucket of ice water to improve chilling, along with a pre chiller.
Yes, I have heard of doing that. It should work just fine but I think the pre-chiller will do most of the heavy lifting.
Think about how a plate chiller is constructed: only the two outer channels would have a real thermal path to the immersion water. On a 30 plate chiller one can imagine any improvement in cooling speed would be marginal at best...
I tried this once with no notable difference. Since I live in Texas I have started to find a better way to prechill my wort before getting it down to temp.