If they are truly motivated, they can probably make it over the 5 foot fence without too much problem. I have a 5 foot fence around the run and they have never shown any signs of getting out. Mine are not fully grown yet so they should be able to fly better than a fully grown Delaware or RIR.
I don't know what motivates some chickens to wander (other than getting away from real or perceived danger) and any time you take your chickens out you are taking a chance, but I'd be comfortable letting them roam inside the 8' and 5' fence until they proved I had a reason to worry, at least as far as them going over the fence. My other worry would be things coming into the yard after them, but that was not your question and you have to be able to evaluate your specific conditions.
Good luck!