my NL-SAS drives are spread all over the place i can't tell the difference, it's NL-SAS so i don't expect a lot from this tier anyway. Best practice recommends to expand specific tier by the same number of drives it was originally created with. Yes, with flare 32 it will rebalance within Capacity tier. If you don't have FAST license this rebalance will happen only when you add the new drives.
I'm working on a VNX 5300 that had some LUN's created in sets of 6 disks. These LUN's are no longer being used and the goal is to reallocate the disks to an existing RAID6 (6+2) pool.  The freed up disks are spread out amongst the various shelves.  For example, I have a set of 6 disks in Bus 1 Enclosure 0, Disk 0-5, and another set in Bus 1 Enclosure 3, Disk 1-6.   Are there going to be any notable performance issues if I expand the RAID6 pool by 8 disks that are not sequential and are spread over two disk shelves; or is the performance difference negligible?  Would the data rebalancing process end up reallocating all the data across the physical disks for optimal performance?  System is at FLARE 5.32.000.5.201.     
Expanding pool with disks from different shelves - performance issues?