I don't think your Signum rims will fit; that uses 5x110 PCD with a centre bore of 65mm.
Mercedes use 5x112 with a centre bore of 66.6mm.
Personally I don't think there's any harm in changing the tyre size a little, but your insurer may feel differently (although if it's new to you, you could always claim that's what it came with).
In theory larger wheels give a higher top speed, but lower acceleration; that's pretty academic unless it's a huge change.
Cosmetically, you might prefer the look of a tire that fills the arch a little more; you can find a taller tire (or a different ET) catches when you're turning, but it's unlikely unless it's a large change.