Hmm...I was under the impression radials could be had that matched the stock height, both front and rear, with these wheels. I also thought the ME880's were all bias ply tires so thats a new one on me.
There are ME880's available in radial, just look at the Metzler main site for mc tires and pull up the PDF "cut sheets"...or just look in the tire size, if its got an "R" in it it's a radial...
I ran an Avon Venom R 180/55-18 on an RC rim 5.5" x 18" rim with the same offset (from centerline) as the stock rim
with no clearance issues
you can make front fender spacers easily and it looks fine, your gonna need them even with a 18" front,
you can buy a speedo drive from boxxenstopp that's made for 17" with a 120/70-17/ but I'm using the same setup on my 120/70-18 and my speedo is within 2%, the drive is about $50
As for the heigth issue, on of the main beneifits of radial construction is shorter sidewalls, it's very rare to find any radial tire in anything higher than a 70 aspect ratio unless it's a very narrow tire......
If you haven't already bought these rims, don't, buy a set of 17's instead, or at least out back, there are no quality "sportbike" type tire available for 18" rear unless you opt for a narrow sport touring tire in 160 or maybe 170 width,
If you want something wider than that then you have to buy a "cruiser" type tire or go with a 17" wheel,
I'm running an avaon Cobra 200-55-18 (notched swingarm) and it's nearly 2 inches taller than stock, about like doing a pumpkin swap and dropped about 400 rpms off the rpms at 80 mph, and it'll still stand straight up from 35-40 with no clutch, just chop, wack and hang on....i'll prolly go with a shorter 200-50 next time tho..