If you search "rev hang" you will find a thread, and there have also been sub-discussions in a few other threads that might show up with that term.
I'm paying close attention to this as it has (at least for now) put the kibosh on my (and a friend) ordering ourselves new 2017 2.5i Premiums with 6MT. I've adjusted to lots of different clutches and shifters over decades of driving, but not sure I want to get used to rev hang (and especially not rev gain - or "over-revving" as you described it). I'm also thinking it could be a safety issue if downshifting in snowy conditions or when wanting to use the engine to slow down on steep grades.
The odd thing to me is that my friend's 2014 2.5i Premium with 6MT has no, or very very little rev hang (and certainly no "rev gain"). So it's nothing an SJ (2014+) Forester "has" to do.
We finally found a manual 2017 Forester to test drive (a mere 6 hours' trip), and it definitely had this "feature" (as Tamika at Subaru of America described it). For me, the 2017 I test drove actually *gained* RPM when I depressed the clutch and went to shift (after removing foot from accelerator). I tend to shift at around 2,500 rpm, or maybe a bit more if I'm getting on a freeway or something. My buddy (we test drove the car together) shifts at a really low RPM (to me), like maybe 1,200 or so. It didn't gain revs for him, but just hung a bit. But for both of us it hung much more than the 2014. Sigh. He thinks he can live with it; I'm not sure I'd be happy with it. Also, he has no plans to ever drive in snow again, whereas I might.
We also both drove a Crosstrek. This was to see if it didn't rev hang, and if so, could either of us be happy getting one of those instead of a Forester. Although I prefer the Forester size, I felt I could be okay with a Crosstrek. Except for not having lumbar support, the seats were maybe a bit more comfortable. And (huge plus!), I could get dark seats in a white car (the light grey Forester seats are hard to keep clean, but you can't get black in a Premium unless you get a black or silver car).
I thought it was fun to drive, and maybe felt a bit more "flingable" (it is lighter). I did notice less power (148 hp vs. 170), but I felt I could work with that. The shifter was more like gliding through honey than the metal notchiness of the Forester. The one thing I think rules it out for me though is that it's at a much higher RPM at highway speeds. I kept wanting one more gear to lower the RPM when just cruising (not that it has to have 6 speeds, but the 5 wasn't high enough).
So I'm back to hoping some ECU re-flash will be announced for the Forester 6MT. Not holding my breath, as they clearly don't sell nearly as many as the CVT, and maybe most people will just adjust to the rev hang/gain anyway. The rub is that I can't find another car that meets my needs nearly as well (or even at all). I'd look for a pristine 2016 but I really wanted the noise reduction of the 2017.
Anyway, I'm subscribing to any threads that mention the rev hang/gain, and hoping to hear something from Subaru. (I called them in early December and one of their engineers is supposed to be calling me back (or someone who has spoken with one at least). Five weeks seems like a fairly long delay though, so I'm not as hopeful as I was at first.)
SB