The reason why the SVX doesn't need one, is because the cylinders fire in a 180 degree phase from each other. A boxer 6 is very unlike any other type of engine, whether it be an inline, V style, or boxer engine other than a boxer 6.
Now, it may give a different sound, I won't debate that, but as far as evacuating exhaust gases, it won't have an effect. That is the main reason an H or X pipe is needed on other engines.
I will say I would like to hear the final result
