BWchiptune can supply a box which will let you switch between tuned to stock called the powergate. But I believe it will show a flash counter issue on BMW's own computer. You're best bet here is to give Jason@BWchiptune a call and see what he would advise.
It is a very hard decision to make when you have a car under warranty and you want to stay within warranty but you want more power. Some BMW dealers offer a remap themselves these days at a cost of around £1000 (so I have been told), you get a small performance increase but as it is done by BMW they uphold your warranty!
hope this helps