Different choices...

Doing the Cobra is the easiest, and could come to be less expensive than yyou might think. If you can afford the insurance then I say got for it.
Supercharging and changin your motor can be fun projects. However, they can take up a lot of time and be complicated. Also, you will have to do a lot more to your car so it can handle the power. If you are mechanically inclined and patient, you could make your car something special.

turbo it

I've been building up everything, but my engine before I decide to unleash the power under the hood. I still have yet to do the driveshaft, axles, tranny plates and clutches, and torque converter. I'm getting there, slowly but surely.

I think that the question is what you are looking for in the end. Build up a six, or go with an eight. The base models alone will be a big difference in what you have left over to work with after insurance and even gas now days.

Figure what you want to do with the six to get to the HP you want, then compare the GT's and Cobra's, and see how much farther you want to take those and how much that will cost you.