Higher octane fuels will give you better performance if the engine is going into knock retard with standard fuel. Although the Omega has knock sensors the engine is probably not in a radical enough state of tune to get into trouble on 95 RON fuel so it probably won't make a noticeable difference. That's my experience at any rate. The knock sensors are there to protect the engine against a really bad dose of fuel or lower octane fuels that are on sale in other markets.
Engines which are turbo charged or higher compression will generally give much more power on high octane fuel because they need high octane fuel to run on full boost or without retarding the ignition.
So, when an Imprezza driver tells you how good a fuel is, don't expect the same difference in an Omega!
I'm sure there are better detergents and additives in premium fuels as against the cheapest supermarket fuel. It's difficult to know what that actually does for you in practice. The marketing hype will tell you it makes a world of difference but I ran my last car solely on the cheapest supermarket fuel locally available and when I rebuilt the engine at 130,000 miles the engine was in decent condition internally and the fuel injectors still had a decent spray pattern.
I changed that policy when, a few weeks after getting my Omega, the supermarket sold me duff fuel and put my emissions light on!
I would say if you're concerned don't run it 100% on cheap fuel, try a tank of Optimax every now and then. If it doesn't improve performance it will, theoretically, at least, clean your fuel system.
One other tip I'd give is, if you end up at a petrol station at the back of beyond where the yokals aren't likely to drive performance cars and they sell super unleaded buy the 95 RON instead. In my other car (which needs super unleaded and has no knock control) I've had tanks of duff fuel in such places because it simply doesn't turn over quickly enough. The additives in a fuel evaporate off while it is stored.
Kevin