Although the average male height in the US is 5'9.5", the more practical average is 5'10" for several reasons.
* 5'9.5" includes the old and elderly (age 60+), which brings down the average height. If you exclude them, that will bring the average closer to 5'10".
* Average height of white and black males (the tallest average heights by race) are 5'10", who combined make up the large majority of the US.
* Employers tend to hire taller candidates, so the workplace average height for males is closer to 5'10".
So 5'10" is the practical average. Then the average range for males in the US is between 5'9" to 5'11", so I can see how someone who is 5'11" (while not recommended) would want to get leg lengthening surgery to get out of the average range and break the 6 foot barrier into the tall range. Here are my takes on the benefits of LL by height range.
Below 5'6" - This is short, so it is most beneficial.
5'6" to 5'8" - Not quite short stature, but quite a bit away from the mean. Very beneficial.
5'8+ to under 5'10" - A little below average. Benefits start fading, but still fairly beneficial and helps get you over the mean and enjoy life being taller.
5'10" to 5'11" (maybe up to 5'11.5") - Average range. Not really recommended and only mildly beneficial in most cases, but if the person wants to do it to get past the average range and enjoy their life in the tall range, then that's their decision.
6'0" or taller - Not recommended or necessary at all, and no benefits with LL. If you can't get that dream job, command respect, or find a date, your height is not the issue. There is something else wrong with you, so find it and fix it.