Thanks for sharing your curation stats... I have given up of trying to catch the maximum ROI as I have noticed that I will lose some of my "manual upvotes" freedom that I want to have... My ROI is around 10% or lower, and I'm satisfied with it... ;)
As of now, I really don't know what the max possible RoI is! But reaching to that level would be like conquering Mount Everest or making some sort of Guinness World Record. I am not after it but racing to it feels like some interesting game ;p
I see some people with manual curation are doing better than me. I do agree that manual curation leaves us with a good feeling in our heart. When the reward split was 75-25, I used to get a feeling of "giving" when upvoting and feeling of "receiving" something when receiving an upvote. But 50-50 split somehow changed that mind set in me. However I still do some manual curation but mostly I am very lazy for it. Maybe it will change after sometime. Let's see.