My 300SD is very slow. 
I've thought about pushing more power out of it in different ways, though really you can't get much without mucho $$$ and sending your injection pump to Finland. I've started to look into gears as a way to get a little more out of the old girl.
Currently, my MB 722 4 speed (which I can't find gear ratio info for) and my 2.88 rear end certainly aren't giving me the best of both worlds, as promised (or the best of one world). I live in the mountains, the car is a dog from dead stop and struggles with hills (and, of course, a mashed right foot offers zero entertainment). Highway MPG is pretty good, but could be better (70 mph = 3000 rpm on flat ground).
Readily available (junkyard) final drive ratios are of the 3.46 and 3.69 variety, which would be nice in town but would blow on the interstate. This is where I think a transmission swap might come into play. There is a kit to put a MB 5 cylinder in front of a GM 700R4/4L60-E. I'm very seriously thinking about using said kit to put one of those (most likely a 4L60-E) into my Mercedes with a fabbed crossmember and driveshaft.
I believe GM will offer three things which Mercedes has left me wanting for. 1. A true 1st gear. The Merc appears to start in 2nd, unless on a hill or mashing the pedal from stop. 2. The beauty of overdrive. 3. Aftermarket.
Part of this idea is based on the fact that my transmission is original to the car, having 670k miles on it. It also behaves like it has this many miles on it, and I'm not totally certain it will last many more years. If it goes, I'm looking at a boat load of cash to get another MB trans that I don't really like in the first place.
Anyway, I think the GM trans with more rear end gear will offer the balls that I'm lacking around town/on hills, lower highway revs, and cost less than a rebuild on the stock transmission.
