I have 5 servers 2 x R710 - 3 x Dl380 Gen8, every server connecting to iscsi storage with 4 nic per server. My iscsi storage is Dell MD3600i 10g.
I connected all server with two Dell 6224. Switches are configured iscsi optimized by using dell documents. Switches are connected by stack cable (20g) MD3600i connected to 10 base-t ports. MD3600 12x2TB disks and configured raid6.
ESX version 5.5 and configured iscsi from vmware documents.
iSCSI network and local network are phyiscally different networks. (Dİfferent switches)
Veeam backup configuration is using direct san access with two veeam proxy. I used 2008 r2 for proxies.
I take backups to QNAP with 4gbit link agg.
I want to explain that, I left my job for last 3 weeks to make performance to our backup infrastructure. I read and applied all documents from veeam, vmware, dell and hp. I reinstalled esx server with dell and hp customized isos. I think that I read all comments, forum posts, documents everything an everything on internet
Think that 3 weeks, everyday 16 hours. 
Problems started with, I buy 3 dl380 and configured esx, and connected to iscsi and network. Before this, my backup processing rate 280mb, transfer speed 408mb, and I wasn't used proxy servers for taking backup.
I shutdown new servers and performance growing up, but if new servers are up, processing rate and transfer speed very slow.
and problem is source bottleneck, for example now, running one job, processing rate 62 mb, transfer speed 109 mb. And a think, backup mode is incremental, this is not problem, reversed incremantal performance is same.
I don't know what must I do :S I make a dream config/hw etc. but now my brain is locked and failed :S