Harry made 36 of the 62 birdies he and his partner achieved, plus two eagles.
Davey, 38, shot a combined +1 total with rounds of 76, 71, 75, 68, 72, 70 which is a fine effort but pales into insignificance compared to Hillier's efforts.
Davey has played the same 12-hour fundraiser with Charlie Hillier, who was 11-under par, but his younger brother's performance stunned Davey.
"It was just phenomenal. He is so aggressive and just goes for everything," Davey said. "He just putted unbelievably well on a slower surface here. He hits another 30 metres off the tee than me. He is just unbelievably long. I am in awe of the guy. I am just an old club pro plodding along playing par golf and getting annihilated by this young 16-year-old.
"This is the third time I have done it. It is just a good idea for a fundraiser and good PR for our club but also a chance for our juniors to give back to the club. Out here we only have one green keeper and the golf club is all about its members just giving."
Davey thanked Trevor Smith, Jim McCrone and Dave Glasgow for driving their clubs around the 117 holes and club members, chairman Steve McManaway and green keeper Michael Homer for their support.