If you pool runs out of space, and a write comes in for an unallocated extent, that write will fail.  I guess you could call that a performance problem.

When adding data devices to a pool, you need to consider several factors otherwise that could cause performance issues.

Every disk in a given pool should have the same number of active TDATs on it.

Every TDAT in a given pool should be roughly the same size.  Sometimes the exact same size isn’t possible due to vault and SFS volumes

A given pool should have a minimum of 8 TDATs per physical disk but not more unless you need additional TDATs to consume the capacity on the disk.  If there is more than one pool per disk, each pool should follow the rule of 8 as a minimum.

The pool should be spread across all available DAs as evenly as possible.