Warranties work differently in different countries. Here, there is a law that says that a dealer has the burden to prove that any
mod has actually caused any damage. So, let's say you decide to do a pulley, but something goes wrong with your windscreen (the infamous cracking)--a dealer can't say, "Well, since you changed the pulley, that voids that warranty on the windshield." Of course, this is an obvious example, but even if something went wrong with, say, your head gasket; the dealer would have a difficult time saying that it was the pulley, and not, say, a factory defect.
Still, having said all of this, I simply have not heard of anything going wrong with a 15% pulley (the
JCW is, I believe, a 14.2% pulley), so it's unlikely that the warranty issue would even come up. My '05
MCS is brand new, and I was a little hesitant at first to change anything. But after a lot of searching (MINI2, North American Motoring, and Roadfly.com), it was very easy to determine that the pulley is a very benign
mod. Moreover, it's very inexpensive (the pulley itself is about $110.00 USD). The ALTA
Cold Air Intake (CAI), is $250 USD. Together, they are a very cost-effective
mod (much more so than the
JCW mods).
Ultimately, whatever you do must pass the "Can I sleep at night" test.
Me, I'm well rested.