Yes, setting max_mpx to 1124 will definately help. The storage system receives requests for 459 concurrent operations in one session, but it will only handle 253, so clients are delayed unless you raise max_mpx.
Which ONTAP version are you on? I see you are using SMB2.1. NetApp has had several bugs with SMB2.1, which are only solved in the very latest version, 8.2.3 (for 7-mode). If you are considering an upgrade, use 8.2.3P1, as it solves a bug in SNMP for use with OnCommand Unified Manager 5.x.
To be honest, I always change it with CIFS running. Most cifs options, including max_mpx, are effective immediately for new sessions. To be sure you have a new session, you must reboot the client though (which could be a windows terminal server).