This may not answer your question(s), but it may help someone else contemplating a larger tire.
The OEM rear tire (150/80-16) is right at 6" wide (150mm = 6"), and the tire sidewalls are about 120mm high (150*.80=120). The actual wheel rim width is between probably 3" and 4", but I haven't measured it. You should get someone who works on Yamaha Bolts regularly to measure the rim width for you (assuming they have a rim in stock), as that is one of the controlling measurements that determines the maximize size tire width you should install on an OEM rim.
NOTE: The rim width is normally 2 or 3 inches less than the width of the tire - and you don't want to go much over that as it may cause adverse handling.
That being said, Dennis Kirk had a Pirelli Night Dragon 150/80-16 which is more like your OEM stock tire. And they also had a 180/65-16 which might also be a good fit (though not necessarily a wise fit.) The 180/65-16 is about the same outside diameter tire as OEM tire, but at about 7" is an 1" wider than your OEM tire.
If you are worried bout clearance, you could wrap a half-inch strip of foam around each side of your OEM tire and see if it hits anything on the bike.
EDIT: Appears Dennis Kirk no longer has these tires, but Amazon.com might.
