There are differing opinions about whether fpr over-run is actually a problem and just how pervasive it is. For my money, it happens anytime you have a fuel pump which is much bigger than OEM (which is a must if you want to make enough power to be fun). The problem is that at idle your engine uses very little fuel and so the rest must be diverted through the fpr back to the pump. The orifice through the OEM fpr is very small and so the pressure in the fuel rail rises do to this restriction. This results in a "higher than base" fuel pressure when the engine is consuming low volumes of fuel. Then, when your turbo spools up your engine uses more fuel and the fpr stops being a restiction so the fuel pressure stabalizes. The result is that the fuel pressure no longer rises linearly with manifold pressure and you can't get a good tune.
The reason that the keydiver chip won't help this issue is that it simply sets the base fuel pressure and then the ECU assumes that you have a rising rate regulator. Since the over-run negates this assumption, the chip won't help.
Now, some people maintain that this isn't a problem and, if you don't care about drivability, you can just tune for boost and you'll be ok. For my money you can either get a quality AFPR with a big orifice or else stick with a smaller fuel pump. I ran an EvoIX fuel pump for a few years and the over-run was tolerable. The fuel trims had no problem keeping my AFRs in check. Then I rewired that same pump and fuel trims got funky. Now I have a 255 and the oem fpr would never handle it.
