Although I can't speak about having the Psoriasis & Sutent I can say that it doesn't surprise me that the Sutent would make it worse. One of the horrible S/E of Sutent is Hand/foot/mouth sores so i would assume that since your skin is prone to Psoriasis outbreaks that the Sutent would increase the severity and or frequency.
My husband had the sores SO BAD that he could barely function. It was hard for him to walk anywhere because the sores were between his toes, on his heels, on the ball of his feet. And thats just the ones that were on the feet!!!
In my experience Sutent sores (as I like to call them) are prone to the feet, hands because thats where your body takes a beating (walking, picking stuff up, any pressure period...) So if your skin is "weak" or "broken down" already because of the Psoriasis than thats Sutent's opportunity to thrive..
I would suggest you talk to your Onc and tell him you would like to have him consult w/ a dermatologist since he (the Onc) doesn't know how to treat Psoriasis and maybe the 2 of them can help deal w/ these lovely things =0)
As I said before I don't speak from experince, but from what all the Dr's have told us about how the "avoid" (yeah right) the sores than this theory makes sense.
Good Luck and I hope I helped a little