I'm not all that convinced that there is an understeer problem..have you tried going around the high speed turns in question, "under power", as opposed to coasting through them?

At the track, I would do my slowing down prior to getting to the turn, then get back on the gas---I could continue increasing acceleration, to the point of "drifting", and NEVER found the car to understeer. In fact, with too much gas--exagerating the drift, the back end would break loose first. I was doing this with the Electronic stabilization OFF...I would like to hear from Bernard if his experience with the system turned on, generated the same balance that I had..he did appear to be cornering faster than me, but I also had Toyo tires on my car seriously inferior to Pirrellis)..
Dan