I am running STI upgraded bars at this point. I noticed a HUGE difference in decreased body roll by going to the "bigger then factory" bars. But those bars are not really big enough to overpower your factory (tiny) endlinks. I put the Kartboys on recently, and noticed a little more stiffness, but it was more in preparation for a much bigger bar setup come spring. With the larger links in place, they will really shine with larger bars.
Case in point: Spoke with a fella that had a front and rear adjustable BIG bars on his turbo swapped STI. But for a couple weeks he ran them on factory endlinks till another paycheck allowed for link purchase. He ran the bars adjusted to full stiff, and liked the way the car felt. Once he got the new beefy end links, he tossed them on and left the bars set to full stiff. He said his first ride he almost took the car off into the woods. WAY too stiff. Haha. He ended up taking the bars down a notch in the front, and down to full soft in the rear. Just goes to show you that with bigger bars, the endlinks now become the weakest link (pun intended). I think its not a bad idea to change them, the Subaru once are known to go after some hard driving, and cost more to replace then Kartboys. But do I think its needed for STI bars. NOPE! If you're planning on getting bigger bars. DEFINITELY get them.
