He does (or did -- see below) things that lie on the boundary between the two at times, but I can't recall anything specifically SF-oriented.
As for what his best work is... well, to be honest, I've not read much of Barker in years. My reading took a different path, and I've just failed to keep up with him, especially when he began writing novels that really seemed to me to be needing a serious trim to the verbiage. So all my replies would only be about his earlier work, and there I was less impressed with his graphic, violent, and bloody tales than those which brought a lot of emotions into play, from horror to pathos (such as "Human Remains", "The Madonna", or even -- to some extent -- "Pig Blood Blues", as ludicrous as much of the latter tale can be).
At any rate, I'll be interested in hearing your impressions here.....