If a GTI is optimised for RON95, it's not going run noticeably better RON98. RON98 fuel isn't any more potent than RON95, it's able to undergo greater compression without pinking because it is less volatile. Octane rating isn't an indicator of energy content, in fact some higher octane petrols that achieve their higher octane rating through the addition of bioethanol are less energy dense than normal RON95 unleaded.
An increase in power and an increase in fuel economy go hand in hand, usually indicating that more complete combustion has taken place. Using RON98 in a RON95 optimised car makes the car more likely to be thirstier because it can't run optimally - the fuel is being ignited at a lower compression ratio and so runs more of a risk of an incomplete burn. You might end up coking your spark plugs by running a GTI on RON98.
