[Twix]

The rear will be fine, the fronts though, they may rub with a tire with a decent sidewall on it. I'm running 40 series tires and at full lock my 19" just touch the inner fender. I'm only lowered 30mm. To run a 20" wheel, and any suspension lowering, there's gonna have to be a lot of work done to the inner fenders. Even at stock height, the X1 has odd shaped wheel wells which may cause rubbing under full lock with a 20" and 40 series tires.

The rear wheels will probably stick out past the fender by about 1/4".

Edit: 1010tires isn't all that great, I use willtheyfit.com. They tell you everything you'll need, ride height gain, gap loss etc.