Yes, definitely. Mine gets 450 MB/s write and 590 MB/s read. Before taking it back in to the Apple Store, make sure that these measurements you are using are trustworthy. Instead of downloading software to benchmark you're SSD, you can just enter these commands into Terminal:
For read speed:
dd if=tstfile bs=1024k of=/dev/null count=1024
For write speed:
time dd if=/dev/zero bs=1024k of=tstfile count=1024
Note that the results will be in bytes/s, so you'll have to convert it (just google: "convert <speed> to MB/s".)
If you still come up with 20-30 MB/s, then you should take it in to a store.