Of course you CAN run a 255 with no reg. But it will be rich when the engine isnt consuming much fuel. Thats idle and cruise, or about 99% of the time spent driving
If you start to datalog and tune the car, you will see this effect easily. If you know what you are looking at you can tune around it, but your tuning wont make sense unless fuel pressure is where the ECU expects it to be. Even a 190 slightly overruns the stock regulator, but its well within the ECUs ability to compensate. It still throws off your tuning efforts, but you can get around it if you know what you are looking at.