Update on this issue, turns out the issue was not related to the IO workload by Parallel processing. Turning Parallel processing off did not resolve the issue.
The issue (as mentioned previously) was the fact that the library element was seen by Veeam.
Tape library drivers were as follows
1. Symantec drivers for Tape drives
2. Vendor Drivers for Library / changer element
Veeam could not see the drives but could see the library element, we had the registry change applied to have long frequency between Veeam polling devices....... Issue seen.
Change the Library / Changer to "Unknown Media Changer", as soon as we did this in device manager Veeam automatically listed the library as "Off line". We could then remove the library element from Veeam.
After making this change the backups ran correctly with no performance issues, so even with the registry change Veeam looked to be "polling" the library element for its status which was effecting Tape performance (Don't know how....)
Hope this helps.
NOTE: No changes were required to the BackupExec Environment after these changes were made.