Thanks for your answer.
Yes, specifically I am only interested to find out how many unique geocachers have found my caches in a specific area. It makes a difference to me whether only one person has found 100 of my caches, or if 100 people have found only one cache each. Ecologically it makes a difference how many people have trampled on the vegetation.
The reason behind this: I like to hide caches but I want to do it in a environment friendly way. I want to balance the difficulty rating against number of visitors to my caches. Therefore I need this metric to base my decisions upon.
GSAK is surely a great tool, but I am a Linux user and have no Windows installation on my computer.
When applying a center/radius filter it would probably be much faster because of the reduced number of caches in that selected area.
Please, if somehow possible, incorporate this feature in the statistics page.