Having any shop do anything without previous experience is a horrible idea. Especially on a unique car like the MINI.
There's a few performance shops, as well as pulley parties that get scheduled, check out your local forum and see what your options are. That way you get an experience person to install your pulley.
The warranty is a time limit that MINI has set and feels comfortable with. It doesn't automatically relate to the reliability of the entire car, more of what customers expect, and what they hope to get. If you change the car, you're moving outside of the warranty anyway, so expecting something to last up to the warranty is pretty irrelevant.
With that said, lots of people have put on a 15% pulley and not had any problems. Especially given how similar the supercharger stock is vs the JCW
one (search for info) which MINI does fully warranty with the reduction pulley. If you're so worried about problems though, you should do a lot of research and searching on here before you gamble, because there's always some risk.