Our sources have at least dual-teamed NICs which are trunked/aggregated at the switch end and both backup servers have quad-teamed NICs.
I've played around with the block & buffer sizes and have seen some slight gains but then the more testing I do these gains seem to evaporate. Currently we're getting around the 1TB per 24h mark to LTO-7 regardless of the block/buffer sizes.
I'm seeing some strange things on our LTO-6 setup too. Two identical jobs, both from the same storage to the same tape 3.16TB - 4d 5h 31m 43s at 1,827 MB/min and then 1d 18h 38m 39s at 2,080 MB/min