Seems pretty realistic to me still pulling well over 1lateral g in the corners and turning and stopping just fine whilst not spinning the rear wheels in an ott manner, considering road tyres.
GT5 makes the realistic tyres feel like you're driving on ice by offering ridiculous levels of grip from the racing tyres. You just have to adjust your expectations, in my experience and it quite quickly feels realistic, or, as I tend to do as much as possible, stay away from the racing tyres all together.
