For hot water, the copper institute's max flow recommendation on hot water is 5fps water velocity. On a 3/4" pipe, that equates to about 8gpm. Cold can safely flow faster, so if you combined the two, depending on the desired outlet temp and the inlet temp, you'd have about a max of 20gpm, which is barely at the needed flow for 8 2.5gpm showerheads...and, that assumes quite hot inlet so you can maximize the cold contribution to the flow. Yes, the pipes can flow more than that, but that goes against the copper institute's guidelines. If your inlet hot water is limited to the recommended 120-degrees and flow velocity...eight showers will end up quite cool if you want to have them all running at the same time, especially in the winter when the cold water is, naturally, colder than in the summertime. Excessive water velocity can do several things: create noises and literally, start to erode the pipe (hot water is worse on this, thus the lower maximum velocity recommendation). When you don't have excess capacity, you don't get the same Bernoulli effect (speed up via the restriction) in the shower head, and it starts to act like a rain shower head, and more like dribbles out. IMHO, you're on the edge with that valve and 8 showerheads. Depends on the effective length of the run (each change of direction adds to the effective length) after it how well it will work.