There are a couple of points in your question that I don't quite understand:
You're proposing to offer "much more extra credit" but "keep the point values low": don't those two things work against each other? In my experience, students don't do much extra credit when they do not in fact feel that it will lead to a higher course grade.
You speak of using extra credit to offset poor midterm grades and also speak of meeting the needs of the "top 20% eager learners". Those are different pools of students.
In my own experience, the second bullet point is a major point of tension for extra credit. Students would like extra credit to be routine work that they can do to improve their grade, possibly even to offset more challenging or time-consuming course tasks at which they didn't excel. But instructors usually see extra credit as a way to reach the "eager learners" at the top of the class and end up giving extra credit that is harder than the normal material. For instance, when I teach freshman calculus if I assign extra credit it's usually the less routine and more theoretical problems. Most students (correctly!) perceive that just doing the standard work would be a more efficient route to a good grade.
I have never seen a course instructor ever give more than a little amount of extra credit. Is there any reason why it would be a poor choice to offer a great deal?
I think that most instructors feel that students who get bad grades on exams and then make it up with ancillary work should not get similar final grades to students who do well on exams. In other words, since many (most, maybe; certainly not all) courses have "mastering the material, as shown in exam performance" as the primary goal, giving too much extra credit works against this goal. Most extra credit is a kind of "homework": I hope that people know that in 2014 it is all but prohibitively difficult to ensure that homework is being done by the student. In an earlier question you asked about assigning Amazon Mechanical Turk-type questions to the students as extra credit. That very terminology raises the objection: maybe the students will use the MTurk website to get others to do their work for them!
But there is no absolute right or wrong answer here: you do get to decide what works best for you. Honestly, I think you're essentially proposing "rebranding" some of the course work as extra credit in an attempt to make the students more excited about it. Could that work? Yes, it could! You are certainly entitled to give it a try. However, since most students do substantial amounts of extra credit only when they are trying to increase their grade, if you repackage too much "normal credit" into "extra credit", then you risk decreasing the amount of work done by the typical student in your course. So you should analyze the incentives carefully.
Finally, speaking of the mind games that get played out in a classroom:
In the past term, my teaching evaluations were full of complaints that I assign too much homework.
This need not imply that you are assigning too much homework!