I don't have an upgraded downpipe, I figured I will change it when/if I build the tranny. But, the most difference that I have read is that it changes the exhaust note at full throttle.
It helps drop your egt's quicker, don't know about exhaust note I have a 5" straight pipe.
It can be installed with the tranny in, its not that bad, hardest part was getting the old one out of the turbo, but with the heads off I bet it would be a lot to easier