Peoples thinking around octane numbers has always puzzled me, in particular owners of performance orientated vehicles. (Please note these are observations I've made after being on various automotive-based forums for nearly 20 years, not directed at anyone here)
Often the performance variant of a model is generally always the flagship in the range, as in the most expensive. I could never understand why someone would buy the most expensive variant, they fill it with regular fuel. Why would you compromise the performance of your performance vehicle?
I couldn't begin to count the number of times I've read people asking if they can put regular gas in their V8 Mustang. Like, why would you do that? You may as well just buy a Camry. The same would happen with Falcon XR6 Turbo and XR8, Ford tuned them to only
just run correctly on regular so that police departments could fuel them with the cheaper fuel. But in the owner's manual, Ford would state that full vehicle performance and economy numbers were achieved on 98-ron premium unleaded (keep in mind Australian RON numbers are rated differently, 98 is the highest pump gas available, which I think is your 93). If you'd just spent the cash on the fast model, why would you use regular?
All of my cars end up with 98-ron premium in them, through necessity (Mustang, XR8, Jaguar) or by choice. The XR6 is the one by choice, it was primarily designed for regular fuel, producing 195 kW (262 hp) and 391 Nm (288 ft-lb). But Ford also designed it to take advantage of higher-octane fuel, so on 95-ron (which I think is your 91), the exact same engine produces 198 kW (266 hp) and 409 Nm (302 ft-lb). Fueled with 98-ron, those numbers swell to 210 kW (282 hp) and 422 Nm (311 ft-lb). So, from a simple change in fuel, you get a 15 kW/20 hp gain. Why leave that on the table?
I actually had to refuel with regular on a road trip many years ago, the only gas station open didn't offer premium. Save for its first fill when brand new, this was after having ran all of its life on 98-ron premium. The engine felt neutered with regular in the tank, it lacking both grunt and overall smoothness. She really didn't like it and I've made sure it always gets premium.
Sorry for the thread hijack, just something I have a "thing" about fuel.