As a general rule for JET, they require intake and exhaust work, as well as a 180° thermostat to work with their tuning.
The JET tuning will also work well with rockers or a mild street/strip cam.
For most S1 cars here, this means fixing any exhaust manifold cracks, welding the flanges on the outside, porting the OEM inner weld, and replacing the cat with a high-flow unit. This will put your exhaust in the best place possible for power and reliability. We used to state the cat only, but far too many S1 cars have been found with cracked manifolds, and these cracks dramatically reduce the velocity of the exhaust gasses in the manifold.
A modified airbox and better flowing filter, or a CAI or FWI will do for the free-flowing intake.
Once this work is done, you'll feel it. Now go buy the Jet, and add it to the equation, and you'll feel more as the JET adjusts tuning for the work you did.