I'm not all that good with metric.. but you dont need to spend the extra money on the 91 octane gas.

A 50km difference between two tanks is not an average. A change in stop lights, driving speed, and a many factors can account for that loss.

Also, its the middle of winter, did your friends account for that? I generally experience a 10-15% (at least) loss in fuel economy based on winter blend/temperatures/etc. Also if your friends were talking about how far they get in a full tank compared to you, did you account for your tank size difference? Or are they talking numbers overall, or in the summer? Did you compare dirving styles, routes, etc? I am not generally surrounded by hypermilers to know if these conversations exist.

What kind of engine flush liquid did you use? an oil flush, fuel additive, coolant flush? If your running 91 octane with a fuel additive, the timing advance the computer would be using to accommodate that change as depending on what you have done you might be in the 94-100 octane range. You at least "compensated" for the 91 octane with platinum plugs.. sort of.

you need a little more info here to get credible advice imho.