The performance requirement for third-party SSDs has been stated in terms of read throughput, where the built-in PS5 SSD provides about 5.5GB/s and third party SSDs may need to be a bit faster to provide the same QoS. This is referring to sequential read speeds, generally measured with 128kB or larger IO sizes. When SSD performance is stated in terms of IOPS, that's usually referring to small block sizes (4kB) and random IO rather than sequential.
(Strictly speaking, the access pattern that matters most for games will probably be something like 128kB random reads, so the attained throughput when a game is streaming in chunks of partially-resident textures on the fly is likely to be somewhat lower than purely sequential reads.)
