I flew SFO-HGH-SFO a few times. There was a 15 min delay for HGH-SFO due to tight air space in Shanghai. With this said, international flights at HGH should have higher priority over domestic flights. Since HGH-SFO flight is only available on MWF each week, you may have to call UA to switch your departure city to PVG (5 flight a day) or to switch to an ANA flight if there is a delay or cancellation. I have no idea about CX's HKG-HGH flight perfirmance. According to flightstats.com, CX5626 (early AM) has 80% ontime performance; CX5622 (late evening), 45% on time, 13% late, 11% very late and 28% excessively late. If United flight arrives at HKG late, you will have a good chance to catch CX5622.
Cheers!
p.s.: I encourage every one to put your travel dates on wiki. If you do not have UA Gold status and above, someone can guess you into UA lounges or call UA elite phone line for you.