Just got back from Columbus, OH. after the install of the SS headers & cats, the Evosport custom 3.45 diff. and the RMS short shifter. The diff. was put in first by my dealer and a race shop put in the headers, etc. because I wanted to use their in-house dyno for pre & post dyno.
The diff. feels great, like I always thought the M5 should feel. Great bang for the buck. I have a 30% fixed lockup and a 2X dynamic lockup. To answer some questions I've been asked about the diff. and increased lockup: first, tire wear (inside wheel drag):
"You will have more tire wear on cornering--MORE EVEN tire wear. With
the current lockup ratio, you have less tire wear on 1 tire only, as 1 is
doing more work than the other. The more even the lockup, the more even
your tire wear will be."
Second, understeer/oversteer:
Oversteer/Understeer:
"With more lockup you will have may have more understeer leading into a
turn. However, this is only if you are flooring it going into the turn. As you
stated on the phone, you wait to accelerate hard until the apex of the turn,
and thus would notice minimal, if any at all, increase in understeer. Even
if you do throttle-hard into the turn, with a 40% lockup there would only be
a minimal increase in understeer. A 75% lockup would be another story...
Oversteer, which again would only be if you accelerate hard into a turn in
wet conditions (which obviously is not recommended) would only be a minimal
change with the 30-40% lockup. However, the way we determined to build your differential, with a 2x dynamic lockup, you would have the power much
faster, and thus could find yourself in a dangerous position--again, only if
you throttle hard into the turn." (Thanks Ben at Evosport for the quoted answers!) Bottom line, for me at least, the diff. is great. The engine is revving a bit higher, but for my beast it corrected some inherent inadequacies.
Next, SS header and cats: All I can say is wow! Expensive, but worth every penny. Top notch ceramic coating, fabulous sound and power galore! I'm still smiling. Tony & I switch cars for a bit on the way home. Tony's engine has always felt stronger than mine, but he feels that 6th gear in mine now feels like 4th gear in his! The sound is gorgeous! The back burble I used to get from the SS x-pipe now is simply succulent! The difference in acceleration is incredible. I did not do a post dyno YET, because I wanted to dyno it after the Powerchip software created for Evosport to account for the headers is installed. I was somewhat disappointed by my pre-install dyno of 338.91HP and 317.53 for torque. I expect about a 30HP increase with the headers and software, but we'll see.
Finally, the RMS short shifter: This replaced my Rogue Octane shifter with WSR. I liked the Rogue a lot, but I still felt the throws were longer than I wanted. Effort was great and shifting was always precise, with no vibration. The RMS shifter is adjustable. I had it installed with the setting as it arrived to me. The throw fells significantly shorter than the Rogue and there is NO increased effort at all... why I'm not sure. The shifting is precise with a nice, prounounced "snick...snick". I am going to leave the setting where it is for now, but at sometime, I may try one setting shorter and see how that feels.
Anyway, I dropped a lot of dough into these mods., but I am very happy with the results.
Supersprint Headers/cats, Evosport 3.45 diff. & RMS shifter installed!
Steve, Congrats! Quick question for you...........
Black/ Black & Grey E60 /// M5 Nicknamed Mighty M5 
)
Good Job buddy. 
Linear Mode
