That creates an angle of 18.21 degrees, which is less than a dog would see in an agility trial for the dogwalk (meaning, it's really not too steep of an angle).
2x8s laid flat are doing to have a LOT of flex over a 16' span. Going this route would give you a 14.5" wide walking surface, which is pretty generous. But the flex that this assembly would have would rule it out in my opinion. I think I'd do something more along the lines of this:
2x8s on edge, with a 3/4" plywood top- 14" wide (or however wide you want). The 2x8s on edge also give you some place to secure railing.
C-ATCH Pimg - DOB: 2/06, RATO, UJJ (x2), DJA, HIT, CA, CGC, High in Games, Reserve High in Standard- CPE Nationals 2014
RATCHX High Jinks vom Neuanfang - DOB 9/12 (Gotchya Day: 1/23/2013) CA, agility superstar in training