I recently sold my BGC stretch for $500 w/ additional in-shroud cooling fan, which was on my 87 T. I really liked it, and gained a solid 3-4 tenths at the track w/ it. I have now upgraded to a CAS V2 for my 87 GN. When I sold the T, it had a stocker, to which the new owner upgraded to a CAS V4. He had no installation problems and really likes it too! He says it feels every bit as good as my previous BGC when it was on the car.
The points mentioned by everyone else are correct, in that, what are your end goals?
If you're going to drive the car on the street for any great lengths of time, and during a hot summer, you would be good to stick with the stock location setup. I believe the CAS V4 will also allow you to run your front sway bar, to which I was unable to do with the BGC version. It also depends on what you want to spend. I believe you can find most stock-stetch versions for about $700 (as opposed to the $850 originally paid), and front mounts run around $900 to start with, not including the radiator and fan upgrades you'll need in order to run it efficiently. Having said all that, what I really meant was,,, go with a reputable stock-stretch and you should be completely satisfied. Check out the used parts sections on gnttype.org and this site. People are always upgrading, and you can pick up sweet deals on used pieces.