It's missing all of the tunable parameters they added for the 5.x drivers. In particular, it's missing all of the IRQ and RSS steering parameters that are pretty much required in order to get anywhere near 10Gbe in the interfaces. The 5.x driver was quite a long way ahead of the 4.x driver with regard to tuning. If you do a modinfo on each driver, you can see the parameter differences. I'm not exactly sure where the Ubuntu/Debian guys *got* this driver, because the current kernel sources have the driver at version 5.1.0 as of May 2017.
On a side note (I know you work a lot on this, Dietmar - I've read through a lot of the git stuff, so I figured you might appreciate this) I managed to crash my storage array today 4 times before I realized what was happening. It's a Dell PS-M4110. It really does not like it when you do low level TCP tuning with sysctl. I actually *crashed* both the primary and backup controller cards on each of my SAN shelves just by tuning TCP on my Proxmox hosts. I didn't even know I could do that. I think it didn't like tcp_tw_reuse, tcp_tw_recycle, and dsack or some combination of the three.
Incidentally, you guys wouldn't happen to have some reasonable documentation for actually *getting* anywhere near 10Gbe out of open-iscsi would you? I'm getting nothing even remotely close. I can get line speed with iperf3, but iscsi is barely scratching at 1.5Gbps. I've tried just about everything and can't seem to make it go faster. It could very well be the Equallogic array itself. I have no idea what else to tune at this point.
