312's do stop you better than the 288's, no doubt, but they just upset the balance of the car under HARD braking with 232's at the rear, IMO. (Nothing dangerous or anything you can't live with everyday, just too much of a difference between the two when driving on the edge, for my liking).
If you've got the extra ~£100 to throw at the 312 carriers, then go for it! 
Ive got the 312mm conversion, its loads better! Cant say ive noticed anything about it affecting the balance under hard breaking, surely this could be said about any car with upgraded front brakes?