I have the Milltek... Most noticeable is the sound which is great, an my reason for purchase, however the car pulls better at higher revs all the way to the limiter, which I hit too often!
...This is normal for high quality exhausts that are free-flowing, due to reduced back pressure. Most see 2-3% power increase. SOME may see a reduction in low-end torque too! However, they do have to be well made, as the tuned length to the collector and the position and type of crossover (if employed) is critical for performance.