This is a common thing, clients thing that Disk is faster then tape when usualy is not and I say this because when you have a VTL or Advance disk you are sharing that pipe with a lot of hosts slicing the throughput for each client, they way you need to measure your performance is in a total aggregate how many streams you allow to run at the same time to disk and add their throughputs, this will tell you that you that Disk is faster then tape, but when you open the pipe to multiple clients each one will get their piece of the pie.
In my opinion 32MPX is way to high, dont know what are your requiriements and how many drives you have available, but you restores will suffer a lot even is posible you cannot restore some data, try to lower that value to 8 or 4, the speeds are the same you will just give more bandwidth to the jobs and reduce the risk.
Hope this helps.
Regards.
