It took me some time, but i did find some head info re Eheim. it will provide some idea of what head does to diminish flow. The below chart if for their 1260 return which has a max. rated output of 625 gph at 0 head height.
At a head height of 4 ft. the flow looks to be about 435 gph which is a drop of about 200 gph or 50 gph/foot. Since at 12'1' is the max head, that would work out to 635 gph/12.083 ft., or about 52.2 gph for each foot the head is raised. That is about a loss of 12% of capacity for each foot the head height is raised. Bear in mind that head height flow rates are a function of the total water weight being lifted (i.e. all the water inside the return hose until it starts to come out). That weight increases with height, but also if the diameter of the return hose is widened.
There is one other factor to bear in mind here. The potential flow rate of the intake side of any filter must be at least as great as return flow rate for that filter. If the return rate is higher, the filter will soon empty itself.
Many years ago in a long defunct fish chat I was taught about pumps by a gent who spent his life working with the really big ones. Hge pums like those in electric power plants or other extremely high flow applications. He explained that flow should always be controlled from the output side, never from the intake side. If the output side of a pump is above the maximum head, the pump will run fine for a long time without any water coming out of the output side. On the other hand if the intake side is impeded and the output side maintains the same output flow, the pump will destroy itself in short order.