What throughput should I expect with MPIO?
Disk writes are actually going to memory (NVRAM) on the filer, to be flushed to disk later - on an idle filer, these will be incredibly fast, and iops of 20,000 are quite believable (you'll see similar speeds from most SSDs).

Reads, on the other hand, need to come from disk unless they already happen to be in the filer's read cache (which, unlike writes, are on volatile memory).

It's hard to pin storage vendors down on iops for spinning disks, but for a 7200RPM drive, 80-120 iops is quite believable. Considering that you've probably lost a couple of disks to NetApp's RAID-DP and/or spares, 2,200 iops is close to what you could expect from 22 disks performing around 100 iops each.

This may not explain your read speeds (your disks may not be doing the full 2200 iops when you're performing a sequential read), but it may at least help explain your write performance.