I rigged up a set of calipers to measure the actual widths of the tires, mounted on the rim, on the bike, with 38psi air pressure.
The 200/55-17 Dunlop D407 eom tire: 199mm.
The 210/50-17 Metzler: 206mm.
The difference is about 1/8 of an inch per side.
I measured the distance traveled in 2 revolutions of each tire mounted on the rim.
Worn Dunlop: 161"
New Metzler: 157"
Conclusions: The Metz is a little wider, but It is smaller in curcumference. I mounted my GPS on the bike and went for a ride. The GPS speed vs. the Speedo reflect the slightly smaller diameter of the Metzeler. With the 200 Dunlop the Speedo was about 1.5 MPH higher at 60MPH than the GPS speed. With the Metzler it's 2.5 MPH. 60MPH on the Speedo is actually 57.5.
My major concern was belt clearance. Would the 210 Metz interfere with the belt? I took the whole rim assembly, rim, sprocket, and brake disk to the shop. The tire guy told me that it looked close for the belt clearance and to pay attention to it.
After putting the wheel back on the bike and alligning and adjusting the belt, the clearance between the belt and the tire was about 3/16ths of and inch. It's close.
The Metzeler has a steeper profile near the center, but is flatter on the sides than the Dunlop. It falls into turns with less effort than the Dunlop, but the flat side then resists going deep into a turn. It's different, but it will work.