We have two 68.7mm Garretts in our 477. They work pretty well up to over 6000 rpm although they are slightly on the small side for the rest of the engine. However, the lag is almost non existent. If you can get the engine to wake up with the twin S400's, you should have no problem with your ET goals. Our C-body runs 8.5 off idle with DOT tires. http://www.youtube.com/watch?v=lUV_PuIySjQ