cant answer that for sure, since i own the 8400. i had the 9400f from a friend for couple of days but i did not compare temps.
i just guess, its almost the same. i dont think the price jump of 8400/9400 to 8600/9600 is worth the minor performance difference. but again, thats my opinion
if you can run somthing like the l9i with a standard 25mm fan, you might be lucky and be fine with an i7. i think there is a thread regarding that topic somewhere in that forum too. but i would look into those solutions, if i would be really in need of an i7.