Rich, there is a bosch 5 bar fpr available that will give linear fuel curve for mapping, have yet to find where to get them though, try bosch motorsport.
also some peope use crushed fpr's, like 4 bar on s3, s4 to give 5 bar, only problem is it will give non-linear fuel curve, is PITA for tuners on RR
what has actually happened here is the fpr spring has been crushed and is constantly more compressed giving the higher pressure but at the same time gives it less movement under compression so non-linear fueling , this is the principle of the adjustable fpr's that are 3-5 bar etc,
hope thats helpful
