If you are going to increase the disc size, dont forget to allow for the extra clearance required for both disc and calipers, inside the wheel cavity.
Quite a lot of people carry out these upgrades then are surprised when the wheel will no longer fit over the disc and calipers and had to go and purchase new wheels and tyres to suit.
Also dont forget to upgrade your tyres too, its no good having a tyre with a crap coefficient of friction and the best set of discs and calipers going, if the tyre isnt going to let you take advantage of the extra braking power.