Re: premium vs. regular...

Higher octane gasoline delays the onset of detonation (or "knocking") which can be very destructive. Instead of a controlled burn at the appropriate time and rate, the heat generated by compression before and after actual ignition causes a spontaneous ignition of the yet unburned portion of the charge. When the two flamefronts collide, you get a knock. This shock can damage pistons, wristpins and connecting rod bearings.

As far as performance goes, I'm not that familiar with Volvos but I know in other applications, the ECU can respond to signals from the knock sensors by retarding ignition until the knock ceases. This happens very quickly and once the detonation is under control the ignition timing is returned to normal. So I suppose that theoretically, acceleration could suffer while the ECU plays with the timing.

Premium gas and octane boosters in and of themselves aren't magic performance bullets. It's what the engine can do with it that matters. I understand that additives used to increase octane also actually slow down combustion which becomes a problem in high rpm applications.

Just MHO.
Erwin in Memphis