I don't have a 500X but it's a heli I have had my eye on since it was released, I just don't have the funds for one at the moment. I always liked the original 500, in fact it probably still is my favorite heli of all time.
The 500X swings 470mm blades as opposed to the original 425mm blades yet the max head speeds achievable in stock form are virtually the same if not more on the 500X! To cut a long story short if/when I do get one I will be running it on 5s 3500-4000mah packs and simply run the throttle at 100% which would still give 2600-2700rpm and probably close to 6 minutes pretty hard 3D, 7 minutes mild 3D and 8+ minutes sport flying or longer if only running upto 2400rpm.
By running lower voltage it naturally lowers the head speed while allowing the motor and ESC to run at a higher thus more efficient throttle percentage. It also means you can carry more capacity for the same pack weight thus increasing flight time. Packs will also remain cooler and last longer and you also don't need high C rated packs, 30C is more than enough.
60% of the time, it works every time!