I don't think you quite see how returnless fuel systems work. Returnless fuel systems need constantly controlled max fuel pressure by a regulator, almost like an old mechanical fuel injection system. This regulator speeds up or slows down the fuel pump speed to keep the mandated dead-head pressure constant, whatever the engine demands are. This is why it doesn't matter how big a pump you put on there; the reg will only allow it to make the mandated pressure, then turn the pump down. A big pump might get there on less voltage but, the end result's the same. This is also why, your tuner needs to change the fuel pump strategy when you go to forced induction and why most demand the installation of a boost a pump or similar.
Why every car has returnless today is because it's safer, heats up the fuel less, creates less vapours for the EVAP to manage and is easier on the system itself, overall. If you look at the lines and rails on this system, they are HUGE compared to a old style return system and can support quite alot of power before they become the choke point. This provides all the volume of fuel the injectors need, at the set pressure, provided the pump can manage to maintain the pressure, which it definitely can, N/A. Generally, when someone adds a boost a pump or similar, they just set the system to run at 17 or 18V constant and nobody reports any issues with doing that so, there is quite a bit of headroom in the stock system.
2014 Premium GT, SGM, Brembo, Auto, Tech, Comfort, etc. GT500s, Steeda UCA, VMP auto tune, 100 shot on BBR tune, best et: 11.38 at 120.85
2012 GT, Candy Red, Brembo, 3.73, Tech, Comfort, etc. Procal tune, Roush upper, UMI LCAs, Steeda red bracket, GT500s, 28X10X16 Hoosier slicks, best et: 11.91 at 115.23 RIP