I have a 1995 SL 500. Idle is rough too, sometimes dropping for a part of a second. I tried a lot to get it fixed, e.g. two new distributors (the electric "rotation" thing???) but nothing helped. My garage says it could be the valves but I should just live with it, as it will cost a lot of money $500 and up...
So I'm fine now, not bothering about it.
I would not spend money on an upgrade, then I would rather completely chip tune the engine (the 500 will run as fast as 280 km/h)