>the speed of our hp surestore seems to be ok, but the amdump takes too
>much time  (6 hours for 23 GB, we want to use now client fast
>compression).

How fast can you get data off of your disks?  If that 23G is one
filesystem, you might consider splitting it up so that Amanda can
schedule it more evenly.  If you're stuck using GNU tar, you've got
filesystem overhead factored in there too.