John, I don't know if you have done anything about the exhaust yet but it really put a smile on my face when I ran a single 2 1/2 in pipe front to rear with an old fashion glass pack about the center and a ugly general replacement muffler on the rear. I have never actually checked HP or 0-60 times but I would say the exhaust added probably 20% to 25% HP and torque. The other thing, before the exhaust change, it just ran out of breath at about 4000 RPM but now it is still pulling strong when it hits the red line at 5500 RPM. My car is actually a US car but it has EURO heads, intake, and FI and runs as good as any 560 I have been in.
Paul in NC

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84 500 SEL (307,xxx miles)