Why would vauxhall go to the effort to installing a bar which makes the handling worse!?
because the design of them is ****..
How any anti roll bar works is by linking (usually via the body shell) either side of the cars suspension with a bar of sprung steel. The idea is that as the car rolls down on 1 side round a corner pressure of the body downwards is counteracted via a crude "spring" secured to the other side of the car. This downward force on the side thats rolling down is in effect translated into a similar acting force on the other side. If this "spring " is secured to the body then that is used as crude lever point and the net result is that excessive roll at one side is largely ironed out by spreading the forces to the other side. The car rolls less which has profound implications to cornering.
However on a Corsa the anti-roll bars are crude affairs and crucially are not secured to the body at all. This means that none of the forces generated are translated thru the body reducing its effectivenss in the 1st place. If you have an uprated suspension which has largely ironed out roll on these small light cars the suspect benefits of the roll bar come more into focus and what has been found is that all it actually does is stop the car reacting to bumps and tends to lift one of the wheels with obvious implications for traction. Which in turn cause's the car to understeer move too.