When you say that you want stiff bars I'm guessing you want to ge rid of the body roll? Eibach and ADDCO atre the only companies with a set of matched bars available right now. H&R should have their bars out pretty soon. Any of these bars are goign to reduce body roll sinifigantly.
The progreass rear bar will help as well and is easier to install then the front ones.
I personally have ADDCO bars front/rear and an FK Hi-tec kit withall my bushings poly. The car is super tight and has almost no body roll.
You really have to ask youself what you want from the car. A great daily driver, a great track car...or somewhere's in between. Personally I'd upgrade springs and dampers befor doing the anti-sway bars. That way you'll know how much body roll your car has.
Another thing to consider, is the value of Technique and Speed when Evaluating Under/OverSteer.
MOST cars will Understeer at Lower speeds.
The Higher the Speed, the More Likely a car is to oversteer with any given combo.
The Trick is a combp of technique, AND Taking into account the average speeds at which you will be cornering.
If you want a Neutral car at 80 MPH, you may well continue to Push/Understeer at 20 MPH in a TIGHT Hairpin.
If you're set up for Tight Auto-X, you may well end up BACKWARDS at 80-90 MPH in an "On the Edge" Corner.
Technique can affect response at ALL Speeds.
Even Pro's have different techniques which lend themselves to Over/Understeer.
You have to find a set-up that's Right for YOU, AND for the Speeds at which you're concerned with "Finding the Edge".
BTW, on my S2, I have the SVT Bars from the Factory, and SVT Springs which I've Added.
MY Car is set-up for more rotation at Lower speeds, so I have Koni Yellow Rear Shocks set at Full Stiff.
I Hope to get Fronts soon, to allow more adjustment.
Right now, I'm definitely a tad Tail-Happy, but not Overly so at most speeds that I see on Backroads.