65mm vs 70mm??

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Unless you are getting a deal on the 70mm, I think it would be a waste of $ and gains would be very minimal at best.

My vote would be finish off the rest of your exhaust if you want your car to breathe better.

A highflow catted midpipe and or a catback would do a lot more for you.
 
Cobra intake inlet is 70mm

The Cobra throttle body opening is actually 65mm.

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.