i tried running my car with xtra injectors and no voltage clamp.. the ECU will pull a ton of fuel, and you will have to retune daily. You want the stock injectors to dump as much fuel as possible to keep the fuel distribution even. By not using the voltage clamp, tuning wil be a royal PITA. For me after a few days with no clamp, i was still adding fuel, it was a never ending process.
FYI- I use the MPI tuner for my AIC and timing control. I also have a FM voltage clamp. I have two xtra injectors before the throttle body.