Okay, by this time, if it were me, and I hadn't seen anything out of the ordinary as a Glock armorer ... I'd arrange to have the G27 returned to Glock with a detailed letter explaining your experiences.
Over on another forum where these pistols are favored, there have been some few folks complaining about some similar G27 feeding issues occasionally. While speculation is just that ... speculation ... and I don't recall anybody ever really coming up with a clear explanation in those instances. Some of them simply got rid of the problematic G27, according to their posts.
A properly clean and lubricated pistol ought to reasonably function. The odds of all three stock magazines being problematic are less likely than it being something with the pistol itself, if the feeding malfunctioning is equally spread across the three stock magazines.
Did the rental G27 seem to be easier when it came to manually cycling the slide, and loading the magazines? The reason I ask is that 500 rounds isn't all that many rounds to have fired ... it's only a case of ammunition. I've fired a case of ammunition through one of my Glocks in a single range session. I wouldn't be all that surprised if a borderline grip stability condition (I dislike "limp-wristing" as a general term) were more likely to manifest with a newer pistol with strong springs, rather than in a well used (rental) pistol with well-used springs.
Nose-dive feeding may be caused by pinched lips, and live-round 'stove pipes' may be caused by lips which have been slightly spread (sometimes caused by trying to load too many rounds in a Glock magazine). If both are happening at the same time, I'd be inclined to suspect a problem with the geometry of the pistol's parts (barrel & locking block) during shooting/functioning, rather than the magazines, and I'd return the pistol to Glock for examination.
It's not uncommon for the 9-round magazines to only easily accept 8 rounds for a while. Mine were all like that until I'd been shooting them for a while, too.
All four rail tabs are present and normal looking, right? I know a Glock armorer for another local agency who encountered a F-series G22 which experienced a left/rear broken frame rail a few months after the pistol had been received and put into service, and it exhibited a puzzling range of various functioning problems until he finally noticed the frame rail tab had broken of the frame. Things happen.
It would be a bit helpful if you were to have someone more experienced with Glocks, especially the subcompacts, try your pistol. How about someone at the range where you rented the G27? Don't they have a Glock armorer (if they're a retail store, as well), or at least someone who shoots them fairly frequently?