I am using Bigpipe and CrashPlan Australia. I didn't experience the slowdown you've indicated. I backed up >10TB at the sustained rate.
I should mention my CrashPlan configuration has been tweaked somewhat. I am using backup sets, with data broken up in to (now) 6-7TB logical cunks to prioritise backups and hopefully reduce memory requirements. I also have increased the memory available to Java, and disabled both deduplication and compression.
Currently sitting at around 40Mbs^-1.
I'm BigPipe and CrashPlan Australia too.
I've got mine in a couple of logical chunks, but I haven't disabled dedup or compression though or done anything with Java.
With Java, what did you set it to. My default values I think are max 4GB if I am reading this right?
uintx ErgoHeapSizeLimit = 0
uintx HeapSizePerGCThread = 87241520
uintx InitialHeapSize := 268435456
uintx LargePageHeapSizeThreshold = 134217728
uintx MaxHeapSize := 4280287232