Did you do any performance tweaking to PostgreSQL with respect to EBS? You have an insanely deep write buffer and quite good random read performance with EBS, which is nothing like the disks people normally deploy PostgreSQL to.
I tuned the hell out of our big postgresql instance a year ago, but I'll be damned if I can remember the rational for every change. I have a list of all the changes from default, but I've long since forgotten/lost the reason for making them.
That being said, we get more bang for our buck by spreading our data across many small databases that don't need much tuning beyond upping the memory defaults. The EC2 cloud isn't great for the uber-server, but it's halfway decent for many small servers.
