Well, after receiving the game today I'm having bad stutters just as expected on my old rig with a E8400@3.3Ghz, 6 Gb DDR2, GTX275 and Windows 7 x64. But when I was having fun anyway (after tweaking the conf.ini a bit) I was buzzing the Kent countryside in a 109 - and I realized that it was not stuttering while flying low over forests etc if i did not see any houses! As soon as a building got in view it started stuttering and as soon as they disappear it runs OK again. To not see any buildings you naturally have to stay at treetop level though over Kent
I tired going to "high" on forests and it still ran fine, IF NOT SEEING A HOUSE!
Try it yourselves... Or it might be hedges, roads or something else artificial?