Here is my take on this.
Having logged Owens GIAC car at the weekend, fuel pressure was logged at a max of 126Bar. My car with all the same mods and Revo S2+ peaks out at about 132.49Bar. Boost on Revo was also higher than GIAC based on boost setting 8 for what its worth.
The OEM fuel return valve is not rated as high as the code commands and the flat spot occurs when the request for fuelling is at its highest. The RS4 valve removes this limitation.
I am willing to bet that the reason for the flat spot issue is that some cars will have a Monday morning or Friday afternoon valve, hence the reason some cars do it and some dont. I am also willing to bet that the reason more Revo cars appear to suffer is because of the higher demand for fuel on their map combined with the compromised valve.
I am actually trying to find a fuelling log of my car with S2 code to see what the max fuel request was vs S2+. As the S2+ code is based around an uprated mechanical pump, and knowing what Revo said about it at the time, it was specifically designed for cars with uprated fuel pumps and the request for fuelling was a little more advanced than it was on stage 2.
Therefore, my theory would be that the issue is hardware related based on demand from software, just as it would be problematic having a standard exhaust/fuel pump on a car with S2+ code. In other words there is no issue with Revo code.
When I get the RS4 valve fitted next Thursday along with the diff, clutch, arb's, WALK, short shift, catch can, diverter valve etc, I will be able to confirm this ASAP. Hopefully it will also get rid of the damned jingling noise the car has had for some time too which I also believe is the fuel return valve shitting its pants
In addition, I will also be speaking to Kev to see if Revo will be doing an equivalent of the extreme file for cars with the uprated valve and a bigger intake - the prerequisites for the GIAC extreme file.