the first post in a thread often gets more extreme votes (scores of greater absolute magnitude) than subsequent posts.

In that the main effect of karma on the reader is to sort posts, comparing scores at different levels of nesting is irrelevant. It is a very biased heuristic to read only comments at, say, karma > 5. I don't know if anyone uses this heuristic. A lot of people only read comments at karma > -4, which probably has a similar bias, but I wouldn't worry about it.

because more people read the earlier posts in a thread and interest/readership drops off the deeper the nesting gets.

People who post replies are serving a smaller audience than people who post higher level comments. It is possibly good that they are proportionately rewarded with karma.