MHDD is showing a few warnings of 250ms or so. Should I replace this drive now? Is it possible this won't get worse? In the past I have replaced drives like this outright, but this time it's BOTH drives in a raid, showing slow spots.

IMHO that's insufficient evidence on which to base a decision. As a minimum, from both disks I would be looking at SMART data, and the shape of repeated read benchmark graphs, looking for either worrying data now, or worrying trends when monitored over time.

As always, you can't be sure that you'll get warning of a failure, and RAID is not the same as having backups - I hope you've got some of those