I've had three dealings with HKUSA in the last few years. Each time they performed the service/modification as promised and returned the weapon promptly. They have always been professional, polite, and reasonable to deal with.
Regardless, I wouldn't worry about it a whole lot. Of my 6 HK pistols, NONE has ever required service (all of the above was optional). These aren't safe queens either - I shoot about 2k a month worth of pistol ammo, not including a couple USPSA matchs and a local club match. Best guess, my USP45 has better than 25,000 rounds thru it and I haven't so much as replaced a mag spring yet. In fact, I've yet to suffer a single non-ammo related stoppage or failure of any kind with any of my HK's. That's going back to 1999.