We do horizontally scale redis as a farm. I'll try to get more details on how we do it as I'm not the one responsible.
We thought of parallel reducers and it does make a lot of sense. The reason they are sequential is to get a first release out so we can juggle ideas with people. If you care to contribute we'd love it. Even if you just create an issue.
Why restrain to sequential reducers when you can parallelize with partitions and sorting?