No, If I was keeping my 15s on all the time whats the point in trying to get bigger discs on? They wont go, at least AFAIK, so I ruled it out. If there is a big disc conversion that will go in behind phase 1 alloys then Im interested. But I just dont think there is. Or at least the absolute biggest you could go would be 285mm which would be skimming the inside of the wheel and if you ever crash it and it puts the wheel out, youre likely to take out the calipers as well and have brake fluid everywhere.
285mm is only 25mm bigger than standard, only half an inch further out for the calipers. It will make a difference but not as much as youd want for the money youre paying I dont think.