[Geezer]

There's generally no downside to upgrading just the rear bar as long as you are reasonable with sizing.

msperber's comments are good but rememeber they are for a race car. Race rubber and smooth tracks can make a very big difference in suspension setup options. Going with big bars for street use can make the rear very loose especially on uneven pavement. Bumps in curves can make the rear end jump around alot rather than stay planted, but the same can happen with excessive spring rates or the lack of proper damping as well.

I always recommend that you do your spring and shock improvements first. The right spring rates can sometimes negate the need for sway bars or upgrading/increasing the size. If after you do the springs and shocks you find you still have too much roll or too much understeer for your tastes, then consider adding sway bars (if you don't have one) or increasing their size.

I went from the stock bar to an SVT bar on our ZX3 and there was a noticeable handling change. Understeer was reduced and the car handling became more neutral. This was the result of a 1mm increase but was also accompanied by the slightly stiffer spring rate increase and firmer shocks that came with the SVT suspension kit. On my RX7 rally car, I too removed the front bar and kept the stock rear bar to make the car more toss-able on dirt roads and the ability to induce oversteer with the gas pedal.

 

[Geezer]

You'd probably find the SVT 21mm bar or a Progress 22 mm bar ideal for your purposes.