Unless there's a substantial gear ratio change in 1st, 2nd, and third gear, I'm having a difficult time imagining how the same car with all other parts being the same, can suddenly transform into a cafe racer just by changing the transmission. What's the final drive ratio in 6th gear? And further to that, how is the MB 4 speed trans is sluggish? Are you referring to the auto trans - I could agree to that to some degree? You only have about 175 HP to work with so even on a good day, regardless of how you chop it up, you still have the same old salad.
Mr. Mafia? What's the rear axle ratio in your car right now? This is where performance comes from and 5 or 6 speed transmissions change that final drive to higher axle ratios inside the trans. I would not go higher than 3.27 and only in a 280SL with a 4 speed gear box. If you can keep your final drive ratio at or above 3.27 then it should be OK. This would indicate to me that your six speed is a close ratio gear box in 5th and 6th gears, or that 5th is 1:1 and 6th is overdrive.