Why do you need to know typical buffer for three variants of LTO? You only have one tape drive, so which version of LTO is it that you have?
I think support were a little unfair. A slow backup does not necessarily equal a slow duplication to tape. It is true that if the backup is large, then the duplication will be large. And if the backup finishes late, then teh duplication willbe late because it cannot start until teh backup completes. But, if the backup is at 5MB/s and finishes late, then on a quietish MSDP pool the duplication to tape should be much faster than 5MB/s - nonetheless, it will still be late.
Duplication to tape, from MSDP, is always an effective single stream and cannot be multi-plexed. This is why it is really important to have reasonably fast disk under MSDP.
.
You say your disk is fast enough. How did you prove this? What random speeds did you get? Are you aware of the recommended minimum disk speeds for MSDP? Would you like some steps/commands/process to test disk speed?