I'd try another brand for a tank or two to see if you notice a difference. The composition (e.g., sulfur content, detergents, etc) can vary somewhat from one brand to another depending on the gasoline retailers' specifications.

No need to move to ultra, unless you're curious as to whether you'll notice any performance increase. Higher octane fuels combust more slowly than low octane fuels; this is to prevent pre-ignition (knock) in high-compression engines. All fuel grades are mixed at the pump from just two tanks, the highest and lowest grades (that's why I suggest changing brands). If you use a below-spec grade of fuel, the knock sensor alerts the ecu which in turn retards the performance of your engine, thus preventing premature engine death from burned valves, etc.

I usually run 93 because 91 isn't available in my area, though sometimes I'll fill with 93, then at half a tank refill with midgrade (89) to get the 91 blend.
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(98 S70 T5SE misc mods, mostly lighting) (92 940GLE)