Luke,
It's not that the mirrors are too 'black' IMO, its because the car needs to be visually balanced.
Black roof and mirrors (on a light body color (added before some smarty pants says they look OK on their black car)), needs to be balanced with more 'black'. Look at Simon's NSXGB silver example, car looked good when he painted the mirrors, but looked balanced once he added the rear R-style spoiler in black.
Your car has black rims at present IIRC. I'm not a black wheels fan. I think you would need to lighten the wheel color if you go with the above changes. OEM silver would obviously work, but maybe the Washi Grey finish that was a JDM option for the R as you are moving into the R look for your NSX?
All the best,
Paul