First off, I would like to state that not all car subs are boomy. There are some designs that easily exceed what is available in the home audio arena at a lower price point. The reason many believe car subwoofers to be boomy, is that... well in the application many are used, they are. It is not due to the subwoofer itself though. It is usually enclosure volume confines of the car ect.
I am not exactly sure about the woofers have ing more distortion just because they are square, I would like some proof. However, I can see how small flaws in manufacturing or design could easily cause non-linearities and the like.
As for the Kicker sub in question, how is it going to be used? That is a specifically designed SPL competition woofer. #1, it's primary design goal is to get loud. Regardless of how it sounds. #2, it is huge, and requires a massive enclosure to get the woofer to perform well. #3, if you are going to be using it in a daily driver vehicle, you would be much better served using another woofer.