Recoverpoint Journal in a FAST Pool?

EMC end-user has two VNX 5300's, and they now want to add RP to their environment.  ALL of the existing storage capacity is allocated into a disk pool, with FAST VP.  There are 2 tiers of storage...  NL-SAS and 15K SAS.  There is plenty of capacity in the pool to support the capacity requirements of their journals.

However, I am apprehensive in putting the Journals in the Pool, even if they were pinned to the high-speed tier.  My fear is that there is no control over disk contention between the production volumes and the Journal volumes  (although, FAST should balance the IOPS across the disks on a daily basis, correct?)

Thoughts, RTFM's appreciated.

Thanks,

Eric