Both the X and the H versions would increase the volume for your GranCabrio. They are both louder than the factory part that comes in the GT / GTC... The difference would be in the tone and volume. The X would be the loudest, it's more of a raw race type sound and would scream a bit more with more crackle / pop on deceleration. The H is a bit smoother sound and not quite as loud as the X version. You'll still here some crackle / pop on deceleration but it will not be as sharp of a sound.
Feel free to contact us directly to discuss and we can help walk you through all the options!
Best Regards,