There the exact same piece...... I have my old H - pipe from my Gt and it fit perfectly in my 94 cobra. Now I have a bassani X pipe and It fits both as well.
To me it seems they use this ploy to charge more $ for the Cobra named parts. I noticed the same thing when I've purchased certain parts from the dealership. I always ask for the part person to check for the GT parts - sometimes the same part number.