Depends on what volume types you are using. For random access - with io2 you can get 256K IOPS per volume, and if you do RAID0, on the largest instance you can get 400K IOPS.
Directly-attached vs over-the-network storage can be fairly different, but going back to a spinning drive is not a fair statement.
You are talking about concurrent IOPS which is fair enough, you can indeed scale that. But every individual one of those IOPS would still have far higher latency than direct-attach storage.
This is a problem when IO operations have to be sequential and can't be parallelized such as when they depend on each other (database has to first read the index to know where the actual data is, then do another IO to actually read said data).
Having lots of IOPS could allow you to make multiple of these queries in parallel (assuming locking/etc doesn't get in the way), but it still means every individual query would be slower than on direct-attach storage.
