I've been wondering that myself, and I did some side-by-side comparisons of video from good laps in both cars. The GT4 has a bit more mid-corner grip in both low- and high-speed corners; the 458 has better acceleration. Braking appears similar. I'd say it's a driver's race between the two cars.
I plan to put this theory to the test next time my wife's at Sonoma. We'll do a little "Ford Vs Ferrari" action and see what's what.
I’m in the process Of making a lexan block off for my GT 350 CAI. My Trackspec louvers are right over the CAI.