6.9 fans, first, are you aware of a Benz site dedicated to your M100?? Find the one that ends with cc.
Now your pipes, yes, I've heard the theory about performance and pipe size on M100s. My very best performing 6.3 uses stainless Tymevalves. As another very knowledgable fellow said, tune it for that exhaust system. Borlas are slightly bigger, I still haven't used them but only because we now make our own systems, all stainless in two cases including headers but the majority use regular steel headers but stainless crossovers and pipes. We use several different mufflers from Tymevalves to now a new devlopment for us, stainless steel glass packs. The best sound by far, the glass packs, no comparison. Performance??? Don't know yet. The air is too cool to judge the recent Glass packs on the Green Hornet to our Tymevalve system mounted on the Silver Bullet. It feels good but only a time slip proves the horsepower. One thing for sure, regular steel pipes and crossovers are a total waste of funds if you plan on driving the 6.9 for 6 years or more. That's the expected life.
On those cross overs; find some photos of a 6.3 system. Note how close to the headers, the 6.3's crosovers are mounted. The closer the better and thats proven. Here in Oregon, we have now found a pro shop that bends the stainless steel making the under the 6.3 view much cleaner than our original stainless cross overs.