After reading and reading about the pulley's... I think I'm going w/ the 19%. So, I remember reading that somebody has had the 15% for ~30K Miles. I'm no mechanic but I'm thinking that adding these pulley's has to be putting stress onto the SC and causing the life of it to shorten. How long do you think and how much loss of power over the time will the pulley cause? Considering this would be a daily driven car and pushing it every now and then...

 

Yes the s/c life will mathmatically shorten; not enough to worry about as long as you don't race your car. I'll say that the 19% is small enough where it's at that physical threshold where it's tough to keep the belt on it at high RPM (this is why no race-car MINI uses a 19% and is reliable).

As for street reliability, there are MCSs out there with near or over 100k miles with 15% pulleys still on the stock blowers.

In general, here's the application differences between 15% and 19%:
15%: Race, high rev street, long-term reliability
19%: short-shifter, street-only, willing to take the risk for that extra 1 PSI

17% might be a good compromise. Please use a shorter belt like a Napa 25-060539

 

Wecome to NAM!
But really there are hundreds of threads on this subject. Just use the search function at the top right of the page.
Ryephile, pretty much summed it up for ya but I would add go with a 17% Helix pulley and then if you bounce off the rev limiter your still not over reving it to the point of damage. Also the Goodyear Gatorback part #4060537 belt is providing me good service.

https://www.northamericanmotoring.co...ight=Gatorback

 

Look at Webb Motorsports under the FAQ and read what Randy wrote about pulley size. He has done more pulleys than anyone. So after talking with him I was confident the 15% was right for me when it came to performance and reliability.