On a M6 car you can get away w/ a CC306 w/ stock heads and the driveability will be decent., however; your hp and tq curves will not be optimal. a cc306 needs breathing room to make power where it is intended to, thus, it gives up some in the mid rpms. I've installed 2 cc306s and about 5 LT4 hotcams. On the cc306 car that had stock heads, the guy still hasn't run under a 12.40 @111 and it definitely is a learning experience to use it on the street. He only has 3.42s so it's easy to bog in traffic and get some cam surge until over 50mph. The LT4 cars w/ similar mods and stock heads go: 12.55@109 M6 w/ 3.42 gears, 12.3x@110 M6 w/ 3.73 gears, and 12.2 A4 w/ high stall and 3.73 gears. As for cars w/ heads, the M6 w/ a cc306 went 11.7@113 w/ a 1.6x 60' on drag radials, all good shifts, the A4 w/ a Hotcam went 11.78@ 113.2 on Z-rated 245/50's.