For my last several batches with S-04, they have attenuated to 78-80%. This seems to be a different and much cleaner strain than it was historically. Like the manufacturer pulled some kind of switcheroo on us.
So I don't think you'll get much benefit from S-04 actually in a co-pitch. If anything, I find the new S-04 to be even cleaner and more lager-like than US-05, so it could be an even cleaner version of the beer than you might get from US-05 alone.