That Seafoam is good stuff. Oversize tires look cool and will lower your rpms a little on the highway. Probably increase your gas mileage even more still. You may lose a little low end with the oversize tire though. I had a 170/80 on the rear and decided to go back to stock. I could tell a difference in take off acceleration with the stock tire on the rear.
Jim on a previous thread another member reported the same results that you have with with the oversize rear tire. I have not gone oversize, and am not disputing your report, just doing the math here.
Mathematically the diameter and the circumference should be almost identical between a 150/90-15 and a 170/80-15 rear tire.
150 mm width X 90% aspect ratio = 135 mm sidewall height. (Tire Diameter =651 mm, Circumference ~ 2046 mm, or 80.55")
170 mm width X 80% aspect ratio = 134 mm sidewall height. (Tire Dia.= 649 mm, Cir.~2040 mm or 80.3")
So the 170/80 tire is actually 2 mm (~ 0.08") shorter than the OEM recommended size.
That`s just over 6mm (1/4") difference in circumference. (Only about 1/3 of 1%. That difference will INCREASE tire rotation for the 170/80 tire by less than 3 revolutions per mile.)
I`m not sure what the tread depth is on a new tire, but a worn out 150/90 will likely be a few mm shorter than a new 170/80 tire. I doubt that most riders could notice a difference in acceleration, engine speed or fuel economy between the two sizes.
Desert Dog, previous discussions on changing tires have noted that the odometer is quite accurate with the OEM 100/90 size while the speedo reads about 10% high.
The 110/90 oversize tire almost corrects the speedo, but causes the odometer to now read somewhat low. One member decided he would rather have an accurate odometer, and just perform the simple mental math of subtracting 10% from the speedo to calculate his true speed.
Many members have subjectively reported better traction and handling with the oversize tires. The individual rider will have to decide for himself what tires he likes best, and whether an acurate speedo or odometer is more important to him.