I switched from a Cobra with the appropriate 65mm throttle body to a Tmoss ported GT40 tubular with a 70mm t/b (the GT40 has a 70mm opening) and picked up 5 horsepower and 12 lb/ft of torque at the rear wheels. I would attribute most, if not all, of the gain to the porting rather than the t/b. Other mods can be found in my sig.
I'd skip it. Might make a bigger difference if your car was forced induction, but I doubt you will feel anything at all by switching. Put the money toward porting your Cobra lower. That would yield better results (in my opinion). The rest of the motor seems set up pretty well.
I'd work at getting your set up to run the absolute best it could before you start throwing other parts at it.
MM&FF did a TB test years ago on a car with moderate mods like heads & intake and they found that the 65mm provided the most hp over stock. The test car actually made less power (than the stock 58mm TB) when going to 70mm and larger TBs.
I am gonna say that the gains your gonna see from a throttle body swap with your current mods are going to be minimal. Get some gears Instead or pick another MOD.