Of course this is just a theory but. I think because of the slower burn rate of the E85, people need to consider piston speed and flame speed. In a high reving motor (ie 8400rpm S65) the piston speed simply outruns the flame. Thats when you get sputtering detonation etc occurs. I don't have the specs, but I think the N54 has a lower piston speed (shorter stroke) than the S65. This could be one of the reasons why those guys can run higher mixes. Also the turbo helps with moving more air.
That being said, I don't know how those HPFP are survining. I changed my HPFP 3 times under warranty on my old 535