Haven't driven the GT Sport rear-engine, rear drive car yet, but if it's like the Yellow Bird in GT6 then it is, and should be, a handful. Rear engine cars tend to understeer when you're on the throttle and oversteer when you lift off the throttle. This can make them especially prone to snap oversteer if one carries too much speed into a corner and lifts off the throttle abruptly.
A lot of people complained about the Yellow Bird in GT6 but I thought it was rather faithfully represented. When I have some time this weekend, I'll put the 911 GT3 through some testing and see how it feels. But I do expect it to be quite lively and to try to kill me if I push it too far.