How do you backup those split volumes? I like EBS volume snapshots, since they're atomic, incremental, fast to restore and make it easy to spin up a clone. But obviously that approach won't work for split volumes.
One thing that I try to achieve anyway, is to spread & "smoothen" the database checkpoint & fsync activity over time via database checkpointing parameters, so you won't have huge "IO storms" every 15 minutes, but just steady writing of dirty buffers going on all the time. So, even if all your files are stored on the same blockdevice, you'll less likely see a case where your WAL writes wait behind 50000 checkpoint write requests issued just before.