I'm guessing you've got the 160/70 X 17. Which means you've got adequate clearance for the slightly bigger tire.
Back when I had my Kawasaki Concours, it came with Dunlop K700 and K701 radials, a tire that nothing else used except for the Connie. At 6,000 miles it had decent tread on half the tire and was down to the cord on about 1/3 of it. There weren't many radials that would fit the Connie, but there were a bunch of bias-ply tires. Over the 165,000 miles I put on two '99 Connies, I never ran another radial. I used Dunlop K491 Elite IIs, Metzler ME-880 Marathons, and my favorite, a Michelin Pilot Tour (or something like that, which Michelin discontinued after I'd used two of them. I could never tell that I lost any traction, wet or dry compared to the lousy K700/K701 combo.
What front tire are you running?
I tried Metzler ME-880 on the NT, and there was an ME-880 in 120/70 X 70, but in the 150/70 X 17 rear size, they only had a bias-ply tire. It didn't last a whole lot longer than the BT-20 and didn't feel as grippy, wet or dry. I probably wouldn't go back to that.