I would say, look for an S55 AMG, either pre- or post-facelift. You're not going to wring much more out of your M113-50 engine than it's already putting out for you without some very expensive work. Now, if you were talking about, say, an M275 V12TT-equipped W220, then yes, tuning would help due to the forced induction. I think there are also pulley mods and other mods for the S55 AMG (M113K supercharged) engine as well. But for a naturally-aspirated engine, you're really looking at a custom turbo kit or a supercharger kit (e. g. Kleeman's) if you want significantly more power out of your S500.