F stop for landscapes w/ em-5

Using that site's calculator, you'll discover that a 16 Mpix camera in m4/3 format becomes diffraction limited just above f/8.  Ie, you should never go above f/8 on any m4/3 lens if your primary concern is sharpness, as beyond f/8 physics dictates that resolution will decline regardless of the optical quality of the lens.

F/8 really is more than enough for any landscape work, as focusing at infinity at f/8 on a 16 Mpix m4/3 camera will yield depths of field (according to this Depth of Field calculator):

7mm:  0.40 m to infinity

12mm: 1.2 m to infinity

17.5mm: 2.5 m to infinity

20mm: 3.3 m to infinity

25mm: 5.2 m to infinity

45mm: 16 m to infinity

You can actually do slightly better than this (have a nearer close limit) if you happen to chose a focus distance at the hyperfocal distance.

The only reason as far as I'm aware to go beyond f/8 is if you are doing macro photography and are willing to sacrifice sharpness of the focal plane (and effective Mpix) to get the whole bug/flower etc in focus.

In practice, a lot of m4/3 lenses show peak sharpness at f/4 or f/5.6.

I'm considering buying the 12mm, currently the sharpest native prime for m43, though it does have the usual distortion issues of a wide lens.  Multiple tests (slrgear.com etc) indicate its sharpest across the frame at f/5.6, where focusing at infinity will yield a depth of field from 1.7 m to infinity, and a hyperfocal focusing at 1.8 m will extend that to from 0.88 m to infinity.