I just went on a DynoJet and had a night of 4th gear runs and a night of 5 gear runs (DCT). Tiny difference - 4whp which could just be the car.
All the other questions you have regarding tie down and tach signal are answered very well here:http://www.m3post.com/forums/showthread.php?t=286347
The shop I was at has only done a few M3s so they found this very helpful, especially for the tach signal. The cover mentioned was a bit of a pain to get back on but it all worked out! Enjoy - looking forward to your results.