Higher pop up reduces overfueling at the whole rev range.After that you have to correct the advance.If you knew what is inside the inector body you will never go that way.On AKE engines there is a pin in the middle and a larger(i don't know how it's called in english so i'll name it plate)The pin is connected to the pop up spring and the plate is connected to the main injector spring.There is 0.1mm diference between them.So the pop up pin is 0.1 mm higher than the main injector plate.When the pressure rises it lilfts the nozzle pin 0.1mm so this is the pop up injection.After that it raises the plate and it injects the whole portion of fuel.The thing is that when you inject the main portion of fuel the two springs are pushed together.Do you get the point.?? So when you raise the pop up you raise the main injection too.That's why the car is weak at low revs.|You don't wan't to do that.Of coarse 20-30 bar is not a problem but no more.Higher pop up pressure reduces overfueling on low revs and less smoke. Worth of testing and maybe your shutdown problems will be over too. I had same kind of problem few years ago that engine shut down when accelerating. There was something with the pump, everything was replaced with ake distributing head. After repair it has been working fine and no shutdowns.