Stick with standard
The bike has been designed to run with a 150 section rear tyre, if you oversize the tyre the natural handling of the of the bike will be badly effected!! The lovely flickability will be lost and high speed stability will be greatly comprimised by the distortion of the tyre on the wrong size rim.
So my advice would be to leave well alone. Besides big fat tyres are for huge speeds and lean angles on sportsbikes on tracks. On the road the standard tyre can match any lean angle or corner speed of any sportsbike with added agility as standard with a 150 section. Thats why the little sports 400's with the 150 section rears steered so quickly.