I'm not convinced that total payout is the most reliable indicator of a "great post".

Not always in reality, but in theory I think that if a post is 'great' (I know that's not easy to define) the author deserves 'quite some' (again it's not easy to name an exact number) reward. :)

What about a cap of $50 ?

I am not a fan of such strict limits, because actually I like the freedom to upvote whatever I want (also if already many other users liked the same post). I would prefer incentives to spread one's upvotes to many different authors, like for example 'diminishing returns', which wouldn't forbid to upvote the same users again and again but made it less beneficial.

Also the ideas I discussed with @blocktrades could lead to less vote accumulation on the posts of 'famous' users.

I agree with you that the rewards for many short posts from 'popular' authors (often witnesses) are way to high, considering the information they offer and their content quality.