If I'm correct you receive curation rewards from minute 27 but not the whole percentaces. (From minute 0-26 the author gets 100% of the curation rewards.)

No, that is incorrect, I believe. For each minute you get 3.33% more curation rewards. After 15 minutes you get 50% and after 30 minutes you get 100% (of the 25% amount reserved for curators, including the super complicated calculation for who came first and when). Many people upvote after 15-20 minutes to catch all the auto-voters for big users you regularly get high upvotes. This can still be profitable under the right circumstances.

This will be changed with the next hardfork down to 15 minutes instead of 30.