Bilal,
I strongly believe that the connection between Stuttgart and Affalterbach is quite strong.
Since the AMG engines have to pass the same durability tests as the serial prod. counterparts I don't think this should be a problem. Furthermore the supercharger is bought from an independent manufacturer, if I remember correctly the current ones are from IHI from Japan?
There are several different manufacturers and variants of superchargers so reliability shouldn't be the issue. Besides that, every engineer will figure out what stress you put on an engine, once you install a supercharger. MB's experience with the four-cylinder engines might have helped in this case!
Just figure out that hp/litre ratios usually tell you something about the characteristic. Every engine with a high ratio, e.g. M3, M5, 360 & 430, S2000, have their peak torque on a very high level. Just for comparison, all of the above mentioned range at around 100 hp/litre.
The 928 you mentioned might produce around 600 hp (?) at the crank in very optimistic conditions and special equipment. On the bottom of the page, 477 RWHP are stated.
In fact every single engine has a slightly different graph. The last one of the 928 looks like the engine is cut-off pretty soon if you consider the graph - it might produce even more hp at higher revs. On the other hand the graph just starts at about 3k rpm, so no clue what the engine produces below that number!
Fact is, that hp is a product of RPMs and torque. If you don't have torque up there, which is best increased by displacement or charging the intake air, you have to increase the maximum of revolutions and enable the engine to produce more hp. There are of course other features that can improve this, like variable-intake manifolds for good torque AND high hp output, "pulsating" air (can't find the right word in English) in the intakes on n/a-engines etc.
Fact is, on a serial production engine, lasting for several thousands of km in different conditions (temp., moist., fuel quality, driveability) demands are set much higher. MB won't be able to squeeze more hp out of significantly lower RPMs than BMW. So 600 hp would mean 95 hp/litre...
The SL73 shouldn't have been much heavier than the SL600 - the std. car was already very noseheavy, so all AMG could do was to minimize this effect by adapting the suspension!
The old C-class had a pretty wooden chassis as well, so I wouldn't even fault AMG or their engineers for that!
Greetings!