Sorry to revitalize and old post but for Oly users could be interesting: I have repeated the test on my E-P5 and it behaves the same way as the E-M5. LOWISO produces 1/3EV higher RAW levels than ISO200 (in terms of RAW levels ISOLOW is ISO250).
Shots at the same aperture/shutter. Left RAW histogram, right camera JPEG:
For this reason, paradoxically the LOWISO camera JPEG has some highlight clipping while the ISO200 camera JPEG (brighter in shadows and midtones) has none.
This is probably how the strange LOWISO is implemented in all Olympus bodies.
Regards