With a gp2 SSD EBS volume you get 3k iops baseline per 1TB of storage up to a max of 10k iops (~3.3TB volume), there is also some burst allowance over that. In my experience so far the baseline numbers have been pretty accurate.

This is a relatively recent change (last year?), it used to cap out at 3k iops rather than 10k. The only real reason to go for provisioned iops now is if you need > 10k iops per volume.