I'm no aix expert, but I will give a cent...

At first: do you transfer 2 TB every day? We are using smf to count the
data and we have much less daily activity than the space of the known
filespaces. We size our diskpool as large as the usual daily backup
amount and move to tape afterwards. If you don't use smf, you may count
your daily new tapes to estimate the amount of new data.

At second: How many concurrent restores do you expect? You need one
tape drive for each restore, for 2-3 drives one SCSI adapter and pci
slot. rs6k's are not as unlimited in io scaling as s390 boxes are.
Think about how many PCI Cards for SCSI (Tapes and disks) and Network
adapters you need.

At third: What about disaster recovery of your ADSM Server itself?

I think when you use a new smp RS6k (F50 or so) with 4 procs and some
RAM you will not reduce performance compared with MVS.