Originally, when I purchased the car, I was told swapping the JCW pulley for a 15% pulley would not make a noticeable gain. I have since talked to educated people who have said otherwise.
Therefore, I have two questions;
1) Is the JCW/Gates belt worth the money?
2) With 55,000 miles on the car, should I replace the belt tensioner while everything is apart?
Are there any other considerations?

 

swapping the JCW pulley for a 15% won't give you too much gain. it will give you more low end torque, and some hp over 4000rpm but noticiable gains I dont know..

you don't need to replace the tensioner.

 

For my '06 JCW, the change from the OEM 11.45% to a 15% was noticeable, not huge, but definitely significant. This change really brought the power down the rpm range, I am very happy with the change.

As my car ony has 13K mi. I am still running the original OEM JCW belt, and tensioner as well of course.

 

I would recommend to change the tensioner. Chad at Detroit Tuned replaces his every ten thou. Its really easy to change. Just take out the bolts and swap. There is no need to lift the engine or loosen engine mounts. I paid $135.00 for it.