Open bug in launchpad.net
[ 7578.057461] TCP: wlp1s0: Driver has suspect GRO implementation, TCP performance may be compromised."
This harmless warning should be fixed backporting  to 4.9 series.
I went over iwlwifi commits since then and I couldn't find any issue regarding TCP/GSO. That said, I agree that it seems to be harmless in that setup and that backporting 0b9aefea860063bb39e36bd7fe6c7087fed0ba87 should help on identifying if it's a false-positive or not.
Someone needs to send an email to netdev@ asking for it, though. kernel bugzilla is not how it's done for networking subsystem. More info in ./Documentation/networking/netdev-FAQ.txt . Thanks.
BTW, I had this same warning with vmxnet3 driver with 4.9. I tried a newer kernel and the message didn't appear. I posted my findings here: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=864642#95 - for reference:
It looks like the warning appeared with this first commit:
but was later corrected with this second one:
The first commit was merged in the 4.9 tree, but not the second one (at least not up to 4.9.106), and was only merged in 4.11.
So I would conclude the warning may be harmless, unless you got it on a >= 4.11 kernel. And I also verified I don't get it on such a newer kernel.
I would close this bug, but it may eventually be nice to backport the second commit in 4.9 to avoid the harmless warning.
Searching on the net, there are other reports with the same issue and with various other drivers.
I added in CC: here the net/ipv4 maintainers I found in MAINTAINERS file:
M: "David S. Miller" <firstname.lastname@example.org>
M: Alexey Kuznetsov <email@example.com>
M: Hideaki YOSHIFUJI <firstname.lastname@example.org>
T: git git://git.kernel.org/pub/scm/linux/kernel/git/davem/net.git