Strong seeded randomness extractors with low entropy loss
I'm not sure if this is what you are looking for, but as I recall, there is a mathematical proof that AMLS (advanced multi-level strategy) is maximal. This document does not contain the proof, but an outline of it is on page 9: Coin Toss

As a practical example, I seed AMLS with timer output to update a hashed accumulator used for seeding a reference PRNG, whose output I then use for research. The output of the seed accumulator does very well on randomness tests on its own, but it is very slow.