Hi There,
That lens should be absolutely fine on the M11 or the M6TTL. Most that use an external viewfinder (I'm assuming you are taking about the x1.25 our x1.4 magnification adapters or the Visioflex type EVF) do so because focusing with the M camera's and longer focal lengths can be a bit tricky, particularly when using them wide open or with shallow depths of field.
Hope that helps
M
M10-R, M10 Monochrom, ex Hasselblad and Fujifilm.