I would just get the good 3M tape. I'm not a fan of silicone use on something like this.

 

What I'm finding out is 3M tape alone isn't cutting it. I just purchased the RD spoiler and I plan to use 3M tape and black silicon to make sure it doesn't blow off the car. I plan to tape it down and use C claps to secure the egdes until the adhesives have had time to cure. That's the best that you can do outside of attaching it with screws.

 

I used silicone for mine. Sits perfect and no sign of it anywhere. Takes a bit more care and time though, but it won't fly off then.

 

heavy duty 3m mounting tape, cost me about $5.50 at Lowe's. It's a small roll, the lining is red and the adhesive stuff is a light-grey color. i laid out towels on top of the spoiler and stacked 3 bricks for each part of the spoiler (12 bricks total). make sure towels are a little rough so they dont slip when you stack the weight, i covered my windshield with a blanket just in case the bricks slipped off.

 

Bricks WTF? No way I'm EVER doing that, knowing my luck of random cardboard boxes falling in the garage and putting a DENT in my door, the bricks would probably fall off.

 

Your M5 deserves better than a replica ACS roof spoiler...poor car.

 

I just removed my ACS roof spoiler today after it being fixed to my car for 5 months and it came of perfectly. I used silicon to attach it and it was 100% solid. It took me almost an hour to take it off thats how solid it was. When ever you decide to remove it, you can use 'AUTOGLYM intensive tar remover' to get rid of all the excess silicon left behind on your window. Took me 10 minutes to get all the silicon off and you could'nt even tell it was ever fitted. Not a scratch or mark left on my window.
I used the same method to romove my ACS boot spoiler and it did'nt damage my paint either!

 

yep, pretty crazy eh. I have the RD Sport roof spoiler...not too sure how different this is in looks and shape compared to the ACS. The bricks laid flat on the spoiler, and wouldn't have fallen off unless someone yanked on the rags.