GTKorea,
I should be able to assist in your query having experienced the tubi 1st hand in a GranTurismo in the past month. Not one but 2 cars. Was suppose to give a short write up but too swamped with work. I'm going to throw a question back at ya - are you concerned with the loudness on the inside or the outside? These have been my observations/experience so far:-
1. If I had a GT I would DEFINITELY be getting the tubi. Stock is way too tame even in sport mode. With the tubi the car sounds like it should be on the outside. Very nice low rumble on start up and beautiful growl as the revs increase. It is not obnoxious at all to pedestrians and still lower in volume than tubi on a ferrari. We did a drive with the Ferrari's and they drowned out the GT's tubi all the way. You dont get unwanted attention due to the exhaust from people on the outside. I still feel the car sound just right on the outside with the tubis.
2. Now we move to the inside and how the tubis sound. Let me be honest it is 'LOUD' on the inside and there is resonance between 2 - 3K rpm. I tried the previous 4200GT with tubis (the so called sport version which is louder than regular) which some people found too loud. I felt that was Ok compared to the tubis on the GranTurismo. If you have rear passengers in the GranTurismo with tubis they are the ones to suffer if they dont like the resonance at low revs. The other GT owner with the tubis said his children had a headache after a long drive due to the resonance. It is better for the driver but everyone has their own preference. Above the 3K rpm evrything is pretty normal in the cabin and you hear the 'humming' of the exhaust nothing too intrusive.
Personally, if I had a Turismo, I would go with the tubi but objectively speaking, the resonance doesnt quite go with the 'image' of the car as a luxury cruiser. Further, as Im not an owner of a GT and did not drive it on a daily basis, I really do not know the impact in the long run. If it was my ride and a daily driver, I may sing a different tune. My description is purely as a short term driver/passenger of the car with tubi. Hope this helps you and any other GT owners in making a decision on whether to get tubis. Lastly, I think the talk of removing centre resonators apply more to our trusty 4200/GS models rather than the Turismo. Nuff said!