Having worked in the speech recognition industry, I'm not surprised. There is incredible complexity going on, and I'm sure they've optimized Siri's performance (so far) for a very limited number of inputs (mics)...mainly the iPhone 4's built-in mic/circuitry. In time, Siri will get better (as a product; not sure about your current use w/a particular 3rd party BT system), but they have to first build on a platform with fewer variables, and then develop it broader as they learn more and collect more real-world data. Potential delays of voice transmission via Kia's BT could be a culprit.

 

She has worked fine for me in the car. I give an extra second before speaking to give a chance for the bluetooth mic to turn on and also speak a little louder. I've been able to text and have her read text without many retries.