I recently bought and fitted the same pipe to my 3.0, so unfortunately not a direct comparison, I have not had the same type of effect with my set-up but was wondering if the spitting and hissing changes when the car is cold and warmed up?
When I did my initial fit of the pipe, just to see how it all hung together, the clamps weren't fully tightened, but tight enough for a quick start up to check for obvious blows etc. When I fired it up I had a lot of hissing from the second clamp (nearest the tailpipe itself). So I tighted it up a lot and now it doesn't really hiss so much, a little when cold but once warm it seems to seal up (expansion maybe???).
Not sure that's any help to you though, I suspect that the combination of mid section, cat and front pipes that you currently have may simply be creating this acoustic annoyance, swapping these items to see what improves it could be costly though.