IMHO, feeding that with 1/2" pipe, you'll never get it to work properly!
The copper institute calls for a maximum flow rate of 5fps with hot water in copper. That equates to 4gpm with a 1/2" copper pipe. Throw in some allowance for cold, and cold's maximum flow rate is 8fps (about 7gpm), you might get somewhere near that through your system. Most 1/2" valves are not rated for much more than 6gpm, and you're trying to feed six body sprays? At what rated volume/spray? Are you also feeding a standard showerhead in addition to the body sprays? A body spray can be rated anywhere from barely a gpm to several. Throw in a 'normal' showerhead, which is typically 2.5gpm, and you could be asking it to deliver MUCH more than the piping will allow. A loop can help, but only if you have enough supply, which, it does not appear you have.
You only get full pressure at the outlets of your sprays when you have more water than they'll let out available in the piping to the assembly. FWIW, the effect of gravity will change the pressure available by about 0.43#/foot elevation change...probably not enough to notice any difference in your case.