Firstly, this question would have been more appropriately posted in the MECHANICAL section

Secondly, to answer your question, they are definitely worth upgrading. Generally you would want the rear bar to be thicker than the front bar, in order to keep the bias towards the rear. One of the main purposes of doing so is to reduce the amount of inherent understeer the WRX is prone to.

A common first upgrade for GC8 WRX owners is to fit a 24mm rear bar and a 20mm front bar. This generally allows the WRX to behave a little more like a RWD when going around corners, simplistically speaking, ie. more willing to turn and follow the corner line.

However, you have got some coilovers so it also depends a lot on the current settings on your coilovers and how they work together with your driving style and purpose. At this point, I would recommend that you discuss this with a specialised suspension place in order to find out what's best for you, your car and your driving. For anyone here to take a wild guess at what is best for you is simply impossible, without knowing which year WRX/STi/Lib/Leg/Forrie/etc you're driving, what suspension mods you've got, what settings are they at, what do you use your car for, how do you prefer your car to handle, what your preferred driving style is, etc.

I think most would agree with me that the man to speak to would be John Fowler at Wheels World in Osborne Park.

Ok, I've filled up my quota of words for the month, seeing as I no longer own or drive a Subaru, or a performance car for that matter.