Thanks for your advice and recommendations, David!
Funny you mentioned "AWS provided a hyper-mega-ultra-super-DB server" - even now they still don't have super powerful sql server instance types, IMO. Luckily for us, we didn't have massive data crunching need for the really powerful beasts. But we are hitting the top limit of their specs, and I don't even consider we have a big SQL server operation.
I was able to use a tool called SQLIO to benchmark the IO capacities in our existing environment and AWS servers. I was very happy to find out with drive sizes big enough that I can match our current IO capacity without paying for extra provisioned IOPS. Those IOPSes get expensive real quickly.
I also found out the IOPS mentioned in AWS specs refers to random read and write vs sequential r/w if anybody is wondering.