It should maintain the RPM just fine.  There's a very slight chance that it'll oscillate, but that's just my professional side intruding into this fun hobby of ours.  I suspect Keith's been there, done that, and adjusted the tuning.

The KR timer has a wire connected to one of the motor leads.  It uses this wire as a pickup for the tachometer that is built into the KR timer.   Therefore the RPM setting is controlled by this tachometer and is independent of battery voltage.   Changing from a 4S to a 5S will not change the motor RPMs.   What will change is head room and AMP draw.  As stated before it is important to match motor KV rating to cell count for maximum efficient operation.