N.B. I first went searching via Google for an answer for this one but got tired after scrolling through six pages of unrelated information.
What was the last year that Ektachrome P800-1600 was manufactured? I have two reasons for the interest: 1). simple curiosity; and 2) I will be working on a project in New Orleans in August and if I can find film from the last batch (at least 20 rolls of the stuff that has been frozen) at a reasonable price I will use film rather than my digital (soon the D4) for the "existing light" portion of the work.
I would stop chasing ether, and find some 400 speed reversal film, and a lab you have confidence in doing a 1 or 2 stop push. That , while perhaps challenging, isshould be doable
As always, trial your materials prior to taking important shots.
I am always dubious about the fast films. Ilford P3200 really only worked well for me at 800.
From the charachteristic curve from a test target I shot it measured for me as a 400 speed film that pushed quite well.