Well I know I am in minority here but since you asked me to say something let me take advantage and share my opinion with you
.
As an Electrical Design Engineer with over 20 years of design experience I believe when a team of expert engineers design an amazing car, they think about every single component and consider every single specification to build a car on a purpose. when I design a hardware, I usually consider some overhead (to satisfy the worst case scenario) to make sure the system is robust enough to maintain unwanted events but there are many things you need to consider before you go forward. Back to our cars, I believe the current S63tu has more capability than 600 HP but it is not just engine. You need to pay attention to the other components including DCT, Differential, Brakes, Suspensions and etc. For example the DCT on our cars is manufactured by GETRAG as 7DCI700 rated to 700 NM (516 lb-ft). Germans are so conservative so basically 20% underrated spec is normal; therefore you can rely on 600 lb-ft if you want to tune your car. I have no idea about the spec of M Active differential, rear axles (as the other parts of drivetrain) or fully aluminum suspension.
If you search over M5post, you can see many people have weird issues including broken axle, slippery clutch, transmission issue or malfunction drivetrain error message and you will be surprised to understand the majority of those cars are tuned!
Here is one example:
http://f10.m5post.com/forums/showthread.php?t=1075579
The thread was started with polling and 168 people participated. over 126 stock car, 100 owners did not experience any malfunction error while over 42 tuned car, 30 of them had drivetrain problem...
You also might be interested to read some threads and get some ideas from people who already experienced them before you pull the trigger. Here you can find some of them:
http://f10.m5post.com/forums/forumdisplay.php?f=438
So after that long rant
If you still really want to tune your car (I would not do that but it's just me) the wise decision is to rely on someone who takes the responsibility of those issues in case something goes wrong; with this I would say stage 1 Dinan make sense for you due to Dinan's warranty ... Stage 2 is too much money with minimal benefit (1-2% ?) which is not significant...
Anyway, Goodluck with your decision and hope you enjoy your ride in my second country
