As a confirmed Minolta user, the 900 suits me: all those lovely prime lenses in my bag give me a head start. I know I'm out of step with most people. I use film, too!
But - the late Barry Thornton, in his book on sharpness, concluded that no zoom lens is suitable for fine art work... Despite improvements, zooms are still a big compromise - you lose out on weight, cost, and aperture, even if the quality is impeccable.
From what I've read, any full-frame digital camera may struggle in the corners, and it's always the part of the field where lenses are least good.
In all honesty, though, how often do you need sharp corners? The odd architectural shot, perhaps... But most shots I take have the main subject around a third of the way across.
Or am I missing the point?