There are many members with ESS kits. Did you see the threads in the E39 M5 Forum?
For example, I have a 2000 M5 with 90k miles - is it worth putting an ESS kit on it? or is the car past it's prime for a superchager mod and I should stick to cheaper minor upgrades? At this point, I am only putting on about 6k miles per year and it's not a daily driver. Thoughts?