I'm sorry for the delay. I was on holidays the last two weeks and thus the support ticket was closed. But we were able to finally solve the problem.
We created a separate network (different IP range than LAN and vSphere management network) and installed two new VMs to use them as Veeam proxies (Win 7 64 Bit / 4 cores / 8 GB Ram). These two proxies are running in "Virtual Appliance" mode (hotadd) with failback to network mode if necessary.
(Now the replication runs fast with processing rates at about 700-800 MB/s and disk read speed within the tasks with up to 920 MB/s.
So in my eyes that's fine and i'm happy now. Next step: scheduling