I just picked up a 2003 mini S with 112,000 miles so the first thing I wanted to do is change the fluids (including the S/C oil) while its off i figured I might as well change the pulley on it. Is 17% safe for a daily driver or should I stick to 15%? Is it a bad idea to mod it now with this many miles?

 

Your milage is a Non-Issue, don't worry about that, and yes 17's have proven to be just fine for a DD, and even the occasional track visit although an I/C upgrade is needed for much track time.
IMO, since you already have a 15% why not just stick with it, honestly no real reason except for the minimal gain for the expence, if you had a stock pulley I'd say absolutely, but just for the extra 2%....it'll just have to be a personal choice.

 

I dont have either pulley right now haha Still got the stock one

 

IMO the 17% is great for a daily driver, with mid level rpm's, with an occasional excursion to the redline. If you plan on doing auto-x, then get the 15% because you will be riding the red line much more. Either way, plan on yearly belt changes IMO.

 

Oops, read it wrong....Yup 17% is fine....