I would recommend the STi shifter. I've tried cheaper shifters in the past and have always been disapointed although I have no experience with Kartboy. No, the new GDA shifter is different than previous models so you can not use any of the older model shifters.
I just put in a Kartboy short shifter (it has really nice welds and machining) and the shift bushings (stiffer than the stock bushings) this weekend. I don't know about the STi model but my experience with the Kartboy shifter is: it is much nicer (IMHO) that the stock WRX shifter (the shifts are about 40% shorter with neglible increase in shift effort). It also has a nice positive feel to it once the bushings are installed (the shifts go snick, snick, snick now...). I don't know what the STi short shifter costs but the Kartboy was $130 with bushings included. Here't the link if you would like to check it out. www.kartboy.com
Overshifting? Not sure what you mean but I know of no shifter in the world that has anything to prevent you from abusing your transmission if that is your agenda. Kartboy has been talked about a lot but I have to remain skeptical until I try it myself. I think I'll play it safe and just go for the Sti unit.
The Hurst shifter in my Mustang has stop bolts to prevent overshifting and bending the shift fork inside of the transmission. Is that just a T-5 thing and not an issue with these transmissions? I have no agenda to ruin my tranny but a little cheap insurance doesn't hurt.