i also have 4 DD4200 that we backup to, our infrastructure is based on c7000. i agree with the comment above, DD should not be used as primary restore storage. if you were to lose a SAN and you needed to restore a lot of vms then it would be a real issue. i am in that same boat but but when we raised this originally it was not flagged as an issue. well after testing i can assure you it is. i used version we originally put version 8 in and are now on version 9.
what i would say is that you should be getting better restore times than that. we get between 25 and 35mb throughput when restoring. in other places where i did not use DD i was getting 80 to 120 so its a big difference. the only difference that i see that you have is we use virtual proxy servers with virtual DD gateways. like this https://helpcenter.veeam.com/backup/vsp ... c_hiw.html
as i said above, my worry is if we lost a SAN we would be in real trouble restoring but to do a few its performs ok. my next task is to look at implementing a landing zone of fast disks in front of the DD and holding 5/7 days of vms and then copy the rest off to the DD for long term retention. also in the next release of DD coming in sept you will see cloud connect, if off site backups is on you list of things to do it looks pretty cool.
another option depending on how invested in DD you are is exagrid. i have not used them but they look pretty good. they have a landing zone built in to the appliance.