My experience;
I haven't driven on any of the available aftermarket ARBs (Eibach and Ultra Racing?). I think they are 18mm and / or 19mm diameter.
The stock 147 rear ARB is 14mm diameter, and very weak. It is so feeble that when I ran the car with no rear ARB (temporarily) I did not feel any change in the cars' handling or steering. I can't see why Alfa bothers fitting it...
I have no experience with the 16mm GTA rear bar, but considering my experience with much thicker rear ARBs (than the GTA bar), I would expect the GTA rear bar to be quite feeble too, just not quite as useless as the stock bar.
I replaced the limp wire (14mm bar) with a 20mm ARB sourced from the front end of a Holden Rodeo (i.e. an Izuzu light truck with a Holden badge on it), very modified to fit the rear of the 147. Initially I left the stock 23mm front ARB alone. This customised 20mm ARB is much much much stiffer than the stock 14mm rear ARB.
As you would expect body roll was very much lessened. There was a substantial reduction in understeer, and a surprisingly minimal increase in 'harshness' (I would describe the feel as 'firmer' rather than 'harsher'). The steering became much more positive in feel and response.
Yes the balance is changed, which is a large part of the reason to "dick" with anti roll bars. The stock set up is very very undesteery. Increasing rear roll stiffness reduces understeer. It does this by increasing the amount of 'weight' which transfers from the inside rear tyre to the outside rear tyre, while simultaneously reducing weight transfer from the inside front tyre to the outside front tyre (this, despite no change in absolute front roll stiffness, but the front roll stiffness becomes a lesser % of the sum of total roll stiffness front + rear).
This is what changes the handling balance when the rear roll stiffness is increased (or if front roll stiffness is reduced). With increased rear roll stiffness and with lateral weight transfer occuring, the inside rear tyre becomes less loaded, so all of the rear weight is less equally shared by the two rear tyres thus reducing overall rear grip. Simultaneously the inside front tyre remains more heavily loaded loaded (weight transfer reduced), so the front weight is more equally shared by the front tyres, which increases overall front grip. So, with either increased rear roll stiffness or reduced front roll stiffness, the lateral grip balance moves toward the front of the car, and understeer reduces.
So, I can see no problems if the stock rear ARB is swapped for a much stiffer unit, while not also increasing the stiffness of the front ARB. I have done this, and it is good. Given my experience, my reccommendation is to fit the stiffest rear ARB you can find, whether or not you also fit a stiffer front ARB.
And more recently;
Initially the customised Rodeo 20mm ARB was not quite as stiff as the aftermarket 18mm or 19mm rear ARBs (according to the numbers spat out by an online ARB stiffness calculator, not actual measurement of the comparitive stiffnesses). This is because my custom rear bar had longer 'arms' than the stock and aftermarket ARBs.
A few weeks ago I further modified the rear bar, welding flanges onto the arms and drilling alternative drop link attachment points (holes) into the flanges, making the effective arm lengths adjustable. The bar is now set up with effectively shorter arms that make it significantly stiffer than the aftermarket 18mm / 19mm rear ARBs. This further improved the handling and steering, with very little increase in 'harshness'. Of course body roll has been reduced a bit more.
This worked so well that I decided to modify the stock 23mm front ARB, by welding flanges onto the arms with alternative drop link attachment holes (much as I did with the rear ARB). Using the alternative attachment points the arms can be effectively shorter than stock, for a significant increase in effective stiffness. At the effective arm lengths I am now using, the 23mm front ARB is now a bit stiffer than the 24mm GTA front bar.
As a result body roll is slightly less again, unsurprisingly. With all this increased substantial roll stiffness front and rear, the car corners very 'flat', though there is still a slight detectable amount of roll. This can be felt just as the car starts to change direction, but once it is actually turning it doesn't feel to roll any further. When pushed in a corner, the increase in front roll stiffness feels to have increased understeer just a bit, but increased responsiveness.
Despite the slight increase in understeer caused by the now stiffer front ARB, I think I prefer it because it feels even more stable on the road, but with excellent response. The car is still waaay less understeery than it is with the stock ARBs.
Regards,
John.