My car also had a steering vibration after the Centa upgrade. It slowly got worse to the point that I wasn't enjoying driving the car and I had convinced myself that the Centa coupling was about to self destruct. Turned out to be nothing worse than a slight missfire due to the spark plug terminal caps (the aluminium bit screwed to the top of the spark plug) being loose. Interestingly, it didn't feel like a missfire - more like the steering rack was directly mounted to the engine! Speaking to Mark Baldwin at the factory he said that this is a common fault.
Spark plug caps are now on my regular pre-flight inspection checklist...
