Is it advantageous or disadvantageous using petrol having high octane number such as speed or x-tra premium?

That depends on the engine's fuel requirement. In some cases such as my VW 1.8 turbo using a fuel with a lower octane rating significantly decreases the performance and milage. If the manufactor recommends lower octane rate fuel there is probably no advantage to using a higher octane fuel. The cost would out weigh any gains. Bottom line, only use it if your engine needs it.