...WAIT...
I am quite sure this is true (for LACP). But maybe you storage layer opens more than one tcp connection.
This is what I did:
Set a bond in Napp-IT using eth0 and eth1 going to port 1 and 2 on my DGS-3200.
Set a bond in Proxmox using 802.3ad for eth1 and eth2 going to port 3 and 4 on my DGS-3200.
I set my DGS-3200 to have Active-Active LACP for ports 1-2-3-4
Set DGS-3200 to Port Trunk as follows:
Trunk 1 = Port 1-2
Trunk 2 = Port 3-4
When I move a full VM disk from local storage to NFS I see 1.7-1.8gbit transmission in "top" and in the Napp-IT GUI under the monitor. I do not see this reflected in the Proxmox GUI, but I assume the GUI is not calculating my bond, but my management port which is eth0.
I have not done any additional setting to my OmniOS as in change the type to NFSv3,tcp.
Please explain what is the difference from what the OP is trying to do and/or how I'm not calculating the transmission speed properly.