Originally posted by Edouard ut I find it lacks sharpness for low-light conditions
IMO, Low light puts many lenses to the test, because there are multiple factors that are not there in daylight shooting. For example, shooting wide open at f2.8 is rarely sharper than with a narrow aperture, but with a narrow aperture you either need longer shutter opening, which risks blurring unless you stabilize the camera, or higher ISO, which can lead to grain/noise especially if you crop.
I think it's true that many great low light shots are actually fairly well lit, but under exposed in PP to give a darker impression. In any case, patiently experimenting with the lens you have in all conditions will give you a better idea of whether it's a problem of technique, camera, or the lens. That's why something line the Single In challenge, using the same lens every day for a month, is a great idea. It gives members a little impetus to shoot every day, in what ever condition they find themselves in. I have really benefitted from doing this with most of my lenses, and I have learned a lot about each one of them.
As for transitioning to FF, the K-1 will recognize an APS-C lens and adapt accordingly, unless you manually override it. Also, I use borrowlenses.com to try out lenses I am interested in before buying them. realistically, owning a lens doesn't mean I'll use it every day, so renting is worthwhile to me, and forces me to try out that lens as much as possible during the rental period. Doing this made me really appreciate the 10-17mm and the 21mm.