In the end, it all comes down to what your budget allows, and what your end goals are. I've recently upgraded to a CAS V2, simply because I found a used one for a great price. Before it, I was running a BGC Stretch Stock location, which hung down lower than a V4 or V4R, but I was still very happy with its performance! I gained a solid 3-4 tenths decrease on 1/4 mi. times, and it pulled hard with it on the highway! The V4 is comparable to this, and does retain the advantages of a stocker type daily driven application, in that, you don't have to worry about overheating the car with it. And as mentioned, many people have run well into the 10's with this setup and the BGC version. I have a friend running 10.6's with his BGC currently, in Houston heat.
My suggestion on such items is to always search the classifieds on this board, and gnttype.org Turbo Trader section, or E-Bay. I've bought many great hand-me-downs like intercoolers, etc, and the price has always been right! I sold my BGC stretch for $500 including an electric 12" fan inside the shroud. There are often good deals out there,,, you just have to look for them.
Any of these will be a major upgrade over your stocker...