Yea but those mufflers are designed to be use with a turbocharged diesel engine.. no telling how it will affect the performance of your N/A gas engine. Diesel engines have lower velocities and temperatures, and those mufflers may be designed to limit/collect and burn Nox as well. It's possible that that could rob performance from your gas engine (vs a gas designed aftermarket) instead of give you a potential gain. Even if you didn't care for a gain, you surely don't want a loss I would imagine. From the pictures I'm sure it will bolt right up, you'd definitely be a genie pig though. FWIW, the only thing that's going to give you an actual tangible performance increase on the gas sky as a tune. A less restrictive exhaust/intake will maximize tuning potential. On the OEM tune it will just move the power band around and give you some aesthetic sound.
Chris - You bring up a lot of good points, thank you for your thoughts. let me try to address a few of your points and everyone please feel free to correct me if i'm wrong.
"Diesel engines have lower velocities and temperatures" - I have to disagree. Diesel engines achieve ignition of the fuel by using highly compressed hot air to ignite the fuel rather than using a spark plug.The air is compressed with a compression ratio typically between 15:1 and 22:1 resulting in ~580 psi, compared to 120-200 psi in the petrol engine. This high compression causes the temperature of the air to rise to 1,022 F
"Mufflers may be designed to limit/collect and burn NOx" - Again, this is not controlled by a muffler, a muffler is simply that, something that muffles sounds. two ways to limit NOx: Diesel exhaust fluid (adblue/blu-tec) into the exhaust stream which then catalytic-ally destroys NOx and Exhaust gas re-circulation, which is done upstream before the muffler.
Gains/Losses at this point is moot. You've already pointed out that tangible gains are from tunes, which makes sense, since it affects the actual combustion process. In this case (Naturally aspirated petrol engine) the exhaust is post combustion and therefore very unlikely to have a major effect, even ill effects are minimal.
I checked with HKS Japan (through Evasive motorsports), and they say it won't work for my car but I'm thinking that its a CYA response in case there are any issues. From a fitment standpoint, I have every reason to believe it will bolt up, the diesel exhaust looks similar, the mid-pipe up turn seems similar, its a 60mm pipe ID, and the hangers look correct. All thats needed is to see if my local HKS distributor will sell me the part, with the expressed warning from HKS that it will not work. I'm sure any exhaust shop would be able to custom fit a new flange or hanger if needed. Will do some more homework and let you guys know the outcome in the next few weeks, it also seems like it will need special order so maybe 3-4 months would be more appropriate as an update time frame