The benefits of fly rods under 9 feet are accuracy, fishing in tight quarters, easier portability, lighter weight, more responsive and shorter fly rods provide more power and leverage for applying pressure during the fight.
Here is a great wright up on the mechanical advantage of a fly rod.http://www.byrdultrafly.com/fishlev.htm
Using the formula in the article above
9 foot rod
8.5 / 0.5 = 17 multiplied times 5 pounds = 85 foot-pounds of power the fish would exert.
8.5 foot rod
8.0 / 0.5 = 16 multiplied times 5 pounds = 80 foot-pounds of power the fish would exert.
7.5 foot rod
7.0 / 0.5 = 14 multiplied times 5 pounds = 70 foot-pounds of power the fish would exert.
In other words the 8.5 foot rod will tire you out less than the 9 foot rod. But drop down to a 7.5 foot rod and the power the fish would exert is dramatically less.