This will make a dramatic difference in the interior sound levels in your mini. Have no idea how this could affect warranty, but then I don't expect to encounter any problems with getting warranty....
I put a nice set of speakers and amps in my mini and had the door and rear side panels and rear seats out of the car for the work. If you have researched improving car sound, you are probably aware of products like Dynamat that reduce panel propagated noise. I couldn't afford Dynamat, but did find some roofing material called "Peal & Seal" for about 1/5 the cost that is basically the same stuff as Dynamat "basic". It is aluminum faced, self adhesive 1/16" thick asphalt product that sticks very well.
I applied this inside the compartment revealed when the 6x9s were removed with several layers over the wheel wells, both inside there and on the outside.
I also put it on the inside of the front door panels, on the inside of the front door skins and on the inside of the door as well as the backside of the glove box and the small panel under the steering wheel.
THe difference in interior noise is DRAMATIC ! The auto volume on the radio now increases the sound level way too much, an indication of how much quieter the car is. The exhaust is also more noticeable because of the lower noise level.
Cost was less than $100 (for a large roll, I only used about 1/4 of it) and a couple days of time.
JC