RE: Aim high, low, or dead on?
At 80 yards with an inline you shouldn't have to worry about it. You might want to aim a few inches lower just to be safe, but I would keep the sights in the vitals for sure. I assume you are sighted in at 100 yards?
It would also depend on what your speed is, if it is really slow it may make a difference. However if you are shooting around 100 grns of powder with bullet that is 350 grns or less you should have a flat enough trajectory that it won't matter.
Where does your gun normally hit at 80 yards?
Paul