All the comments are good.
The XT660 engine only needs 95 RON according to the spec so this is all you need.
If it had knock sensor, then the engine management would optimise the ignition timing for the grade of fuel you use; which in the case of 99 RON, it could be beneficial.
However there is a twist.....
In the last 10 years or so, fuel companies who have started producing high octane fuel (think shell & BP), have been putting special additives in to improve engine running & clean at the same time. So you are not only buying a higher octane but also a chemically more advanced fuel.
So you may not be wasting your money when you buy premium fuel for your XT.
The best bet is to try it yourself. Run 4 tankfulls of your specific fuel & see what you get