Not sure that I understand why you would consider an SMG or Tiptronic (which is an auto) gearbox to be better than the Speedshift auto box on the AMG's. This auto box has a number of distinct advantages over the normal auto box on a regular Merc.

Driving a car with a good TRUE Manual gearbox (i.e. clutch and gearstick) offers a level of control, interaction, and driver satisfaction that is simply not available from any other system IMHO. Not necessarily quicker mind and in most cases actually slower than especially an SMG, just more enjoyable.

If you consider that other "Swiss Army Knife" of cars, the M5, then that car did simply did not sell in the numbers that it deserved BECAUSE it did not have an auto box option and that it what most potential buyers wanted.

Putting a manual gearbox in an SL55 would COMPLETELY change the driving characteristics of the car and I suspect these changes would appeal to a far more select audience. For example, think about how heavy the clutch would need to be to deal with the 700 Nm of torque that arrives at 2,650 rpm.

There's a reason why (even in Europe) more than 65% of Ferrari's and Lamborghini's are sold with SMG gearboxes. It has FAR more to do with the "auto" capabilities of the box than anything to do with it's manual characteristics.

The SL55 is designed as a Cruiser / Bruiser, not as a sports car. As such an auto box (which still allows certain "manual" control and lock up in each gear) is more suited to that car.

For reference, the text below summarises the differences between the AMG Speedshift auto and a regular auto box

"The AMG SPEEDSHIFT® 5-speed automatic transmission with Touchshift combines all the advantages of a manual transmission with the benefits of an automatic. For example torque converter lockup engages even in first gear, while faster gear changes and optimised shift points provide extra agility. AMG SPEEDSHIFT® causes the transmission to stay in gear when cornering at speed, to improve acceleration out of the bend. And its active braking downshift function enhances the braking effect and at the same time provides the appropriate gear for pulling away as soon as the braking manoeuvre has been completed."