While on the topic of premium fuel, I just want to post my own findings.
In Australia, we have unleaded (91RON) and premium unleaded (98 RON). I tried both fuel. The fuel consumption of normal unleaded is about 15-20% more fuel and lack of power. After taking this into account, I think the premium unleaded fuel is the way to go. Well, at least for the W140.
My 2 cents worth.