semilog
curmudgeonly optimist
this is not a lens question, it is a question of which systems perform best with which lenses.
"Best" is a qualitative descriptor. The only interesting question, in my view, is whether a given system can, in the hands of a given user, provide results that are satisfactory within a given application. That's the only "performance" that matters. Without knowing the specifics of the user and the application there can be no "best" system.
Even in the blandest technical terms, specific technical attributes might be "best" but the problem of imaging system performance is multidimensional. There's flare, color rendition, astigmatism, coma, spherical aberration and field curvature, geometric distortion, focus shift, decentering, vignetting, lens size and weight, ease of focus, color shifts, out-of-focus rendering etc., etc. These are all terms that have widely agreed-upon meanings and they all describe important aspects of an optical system's function.
In many applications sharpness, and especially sharpness in the corners, is least among these parameters in importance. Sharpness is relatively easy to measure (at least in the center of the field) and for whatever reasons, in the eyes of many enthusiasts it takes on an outsized importance. In most applications, corner sharpness simply is not going to be the sole -- or even most important -- determinant of what's "best." To take just one example, for me resistance to flare is often much more important in a wide angle lens than corner sharpness.
So, if you genuinely have questions about using the X-Pro with adapted lenses, why don't you tell us what sort of subjects you shoot, or what you want to shoot, and under what conditions? Sports aren't portraits and portraits aren't street and street isn't architectural interiors and architectural interiors aren't landscapes. Do you use a tripod and shoot at ISO 100, or do you shoot handheld at ISO 6400 or even (as I was doing last night) ISO 20,000? Are you more interested in color, or monochrome? What do you intend to do with the files? Which lenses are potentially important to you? What mode of composition do you prefer, and hence which sorts of finders appeal? How important is system weight, and size? How important is weather resistance? Do you have an established digital workflow that new equipment needs to be compatible with, or are you willing to adapt your workflow to accommodate new gear? Do you have really strong opinions about how a camera should feel in the hand, or is that less important than other parameters? Do you want or need to impose financial constraints that will limit your gear options, or is price less of an issue for you?
Once you've answered most or all of those questions, a particular lens and camera might be "best." But until then...