I am no gas expert, but this is what I understand to be the case (and it's probably more than you wanted to know): You've have two things going on when you get higher grade fuels - higher octane rating and usually detergents and other additives that your engine might like. Either one can make your engine run a little better. However, running higher octane than necessary won't make it run worse, ie. your engine doesn't really know what the octane level is, unless it's too low. Engines do not get more power from higher octane fuels (Most people think they do) but instead they get it from the advance in timing that higher octane fuels can support without detonation and/or from higher compression ratios. Since your compression ratio can't vary itself, we'll say nothing more about it. Detonation needs to be defined, because it is often confused with pre-ignition. It is not the same thing as pre-ignition, but it is the same thing as knocking. When you increase the timing, the spark occurs earlier in the stroke. There is a point at which the advance will be too much and cause detonation. Detonation occurs when the mixture gets a spark and it is not sufficiently compressed enough to burn at a regular and controlled rate. Instead it flashes off, it detonates, burning much faster than it would have a few degrees back. This creates a full pressure condition in the cylinder, but the piston is not at the top of its stroke and the engine forces it through and against this wall of pressure. You hear a knock, or worse throw a rod. This is very hard on the involved piston/rod, etc..

A higher octane fuel is less volatile than a lower octane fuel, and thus more stable, allowing more advance without detonation. Similarly, a vehicle requiring higher octane fuel can be set up to run lower octane fuel by retarding the timing - and a loss of power will result from the loss of advance. Running lower octane fuel without retarding the timing may cause detonation depending on how close to the "line" the timing is set.

Another interesting observation is that 87 octane fuel produces more power per unit volume than 100 octane fuel (more volatile = more power). But you won't see that power in an engine because the 100 octane fuel will allow for both a higher compression ratio and more timing advance and thus way more power - but it's not technically attributable to the octane level of the fuel.

So how does this relate to your question?
1. The higher octane fuel is not causing your engine to run worse.
2. Ignition timing and the octane level of your fuel are related, as above. I know nothing about a C180, but if the timing is adjustable, it or some part of the system that controls it, could be your problem. If it is not adjustable and you are using fuel below the minimum octane requirement you could be causing the problems by doing so.
3. If you're having trouble reaching 100kph, you've got some kind of serious problem. But depending on how computer and emissions controlled a C180 is, it could be any number of things - none of which I can even comment on. If I were you, I'd take it in for service ASAP and hope that it is a timing issue.

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1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick