My guess is that it will hurt UA as the connection times in LAX will probably be better than the connections on UA, plus it will be a proper 3 class 744, so you'll get a bed in F.
Interestingly UA is now showing UA815 as ORD-SYD as a through flight, although of course you have to change terminal, as I assume this brings it up on the travel agents systems alongside QF.
UA815 ORD-LAX-SYD 1800-2017/2240-0610
757/744
QF94 ORD-LAX-ORD 1940-2140/2345-0725
744/744