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upex already gives the largest upvote compared to any upvoting services as of today

20X your delegation amount

there is no rewards for curation , however we are working to give rewards whenever someone does not posts

Will there be any additional offers or benefits in the future for the initial delegators?

20X upvote is the most profitable deal I think

The only liquid steem that is paid out is for those that have overdelegation, IE they have delegated more than can be covered by a full 20x upvote. At the current roughly 2.3 million sp total delegated, the overdelegation payments kick in after a user has roughly 115k sp delegated. Hope that helps! :)