In my testing the primary bottleneck isn't about writing to the same partition. Lock coordination to increment the serial key, then send it back across a connection was the real culprit. Virtually all databases make updates into RAM and to a write ahead log, which is then spooled to disk in the background. No matter which pages are written or updated, the changes all go sequentially in order into the WAL. For me, getting rid of Serial Keys was the big hot spot in processing. YMMV.

It's been a while since I've touched db stuff, but pg still has to write the indexes before commit completes, no? So it's still having to go load and write a gazillion pages. And then unless your filesystem has atomic page writes like ZFS, it also writes the whole page to WAL to ensure that it doesn't do torn writes. Writes to the same page within a checkpoint don't need full page write, so benefit.