In heavy write dbs, we see performance drop when oplog is enabled (Master-Slave, MS or RS).

In case mongod would allow options f.e.

--sendoplogserver [server:port] --sendoplogdb [db] --sendoplogcollection [collection]

this would help the master database to focus just on application needs (to write data) and less overhead would be needed for local writes of the oplog.

Assuming 4 heavy-write servers. It would be cheap to build one slow server on fast network to receive ALL oplogs from the 4 servers.