I'm not familiar with those tires, but that sounds like a lot of pressure. I ran mine at about 38/42 on the stock runflats and had no issues under braking or anywhere else, other than the expected understeer in long corners.
It took me a session or two to get used to the car, coming from a fully prepped E36M3, but I found it to be pretty capable.
http://www.1addicts.com/forums/showthread.php?t=226761