I was using the M3-GTR for the Hot Lap Challenge, you should see how this tune performs. I got a 7:02 with the tune I have now, it is quite different then the tune you listed here.
I'm still adjusting to the lack of ABS and my lap had lots of tire spinning in it, so i think a sub-7 minute is possible. And the 2nd fastest time in the HLC is in the M3-GTR also!