the compression is not that high on this engine, using an octane rating too high will result in a very slow burning fuel and as the engine is a ULEV already, its got a boatload of spark advance dialed in, which is killed by the EGR function as you drive along
What happens is that you get excess carboning up due to the slow burn which when sucked in by the EGR gets nicely coated and clogs the EGR completely - then you get a machine gun type knocking problem and drivability issues.
The manual you are showing is for US market and the mention of 87 octane is there because its the most abundant fuel in the USA. My own nissan altima also has this statement in the manual and anytime I put in 91 octane I lose fuel mileage and the engine performs bad. Its best to use good detergent quality petrol in it and at the correct octane rating. - You will hear knock if you use anything less than 87 octane - which you dont have in the UAE.
btw - go back to your dealer and ask him for official GM declaration to use 98 octane, it will be a nice chat session where he will give you imaginary stories