Just to clarify some issues:
there is a noticable degree of stiffness between the 19mm AWR bar and the 21.5mm bar...A bunch of guys on the pclub tried both and commented...But there are material differences between the AWR pieces and other brands...sway bars are made with torsion properties similar to how springs are made, but without knowing the torsional rigidity of the AWR bars or SRmotorsports or whiteline bars, it is difficult to compare...The AWR 19mm very well could be significantly stiffer than other 20mm bars, or the other way around...It just depends on the alloys and materials used...
With a larger rear swaybar, you bias the traction limit towards the rear end of the vehicle...The advantage is that the car is seemingly easier to rotate into and around corners...it also helps with straightline driving, in that it keeps the rear end in place rather than like a rope in the wind (hold a rope with about 2 feet of slack, and hang it out your window at 60mph...that is the comparison I was stabbing at)...
The disadvantage is that you largely change the understeer/oversteer nature of the car...With the 21.5mm the rear end will give out first...that has been proven by about 3 accidents, and 20 or so close calls...and this applies especially in the wet...Some guys were getting scary rear end behaviour simply be lifting the gas midturn on wet pavement...If you are prepared for the changes of the car, you can compensate...
this pertains mostly to a stock P5...the wagon's have the closest to 50/50 weight balance of any of the third gens (not by much, but enough to make a difference...it is still has 60+% in front from what I remember)...The MP3's/MSP's already have a bigger front bar, so the 21.5mm won't bias the traction limits as much...and the ES/LX/DX sedans accept this mod well because of the already large bias towards understeer...with the 21.5mm you get the handling pretty close to neutral...The P5's take it well to, but with the 21.5mm rear bar they become the most tail happy of the entire group...the weight over the rear end from the hatch and longer roof (this has little to do with the overall weight of the car, but more to do with where exactly the weight is located), accompanied with the stiff rear bar could get scary with bad tires in back...
I am not trying to be overly cautious...I have just seen at least 3 guys smack their cars up with a AWR 21.5mm rear bar, whom all oversteered...and a ton of other guys almost lost it...I always recommend checking your tire condition too before installing one..especially on the rear...