With the 255/60/r15 MT street R, 4th gear @ 6k rpm will be around 100-105mph(27in tire)
With the 275/60/r15 MT street R, 4th gear @ 6k rpm will be around 105-109mph(28in tire)
That 275 might be a tight fit. But the tread with is 9.50 inches and should fit on a 8.5 wide tire...