Hey there with my research I hope I can help you. I finally just sold my '69 LeMans, but before I started the process I decided I wanted another muscle car, but I also wanted, A/C, cruise, fuel injection, overdrive trans--the works. I wanted a real street cruiser that was still fast and decent on gas and this was it. I wanted an 86/87 but I couldnt' afford one. Now there is an 85 for sale near me and I have done research on that. Now to answer your question. 86/87 have a lot better intake manifold design over the hot air cars. Besides that the turbo on the 84/85 is smaller than the stock turbo on the intercooled version. The hot air flows roughly 390 cfm. Besides the bigger turbo and better flow, the intercooler significantly increases power also. The heads and exhaust headers also flow better on the intercooled version. The hot air cars get a nod when it comes to brakes, however. Both are SFI, which is a great step up from carburetion which the 83 T has. I would get at least an 84 or 85. Not sure the turbo size of the 83. Hope that helps!