if you neck it down to 40mm then you're only saving ~40g....and then you have to worry about where the holes for your prop screws are going to end up.
with the rear of the hub at 32mm like it is now you would not want to drill past your 18mm thick flange and that means messing with blind holes.
easy way to avoid the mess of tapping blind holes for the prop screws is to go all the way through (front to back) with the 5mm hole and then from the back of the hub counterbore with a 6.5mm bit leaving about 15mm of 5mm hole at the front for tapping for your M6 screws. to do this with a 35mm bolt circle you would need the hub to be no less than 44mm......may as well leave it at 48mm for the whole length.