Yes it "must" be enable for the multi not to drop back to x20 after S3. Just realize it after update to f7d and load default settings which enable the eist by default.
It kind of habit to turn power saving off when oc. I think you can try it on ud4p since it the same issue.

I dont know ; it probably bug since the eist broken after after certain bclk. If I am not mistaken dfi ut board also must enable the eist in order to use the turbo but it already known long ago.