Have now put a few hundred km on with the headers installed and they've developed a nice bronze-ish tinge. Looks pretty cool.
Performance wise, can't say I notice much of a difference. The engine does rev quite freely but then again I wasn't really complaining beforehand either. My butt dyno isn't awfully well calibrated and I wouldn't have realized it was 30hp below stock power numbers until we put it on the dyno. Gonna try address a few more things and do another power run at some point to compare.
Having driven a bit monitoring fuel trims the LTFT's still keep jumping between 5.6 - 6.3 - 7.8 - 8.6 for both banks so doesn't looks like it's made a difference there. Would have been wishful thinking to expect it to have done though.
Didn't notice a massive difference in sound either. It did make the tone somewhat deeper but the difference is very marginal and it still sounds quite subdued to my ear. At idle there is no difference at all, at around 4-5k RPM and above you do hear it more but not as much as I would have expected. I was hoping for a more throatier tone but with the rest of the exhaust remaining stock I guess the muffler does what mufflers do. I want to retain the secondary cats so as to not smell like a bag of rotten eggs so might look into getting an aftermarket muffler at some point instead, but that will be sometime in the distant future.
Video #1
Video #2
Should have really made the comparison a bit more scientific and taken some 'before' videos and maybe a dB reading but of course only thought of it after the exhaust was already off the car.
Here's a brief summary of my experience with the Malian Exhaust RHD headers:
The flanges at the engine head end are perfectly straight, fitment is good and they bolt up nice and straight.
The flanges on the exhaust side are horrible, it's like they never fitted them onto an actual car before rushing them into production - the angles are wrong, the lengths aren't even and the whole slotted bolt hole design is baffling - even if they did line up and weren't hitting each other when bolted up to the head, I have no idea what guarantees that the tubes remain in proper alignment. They had a previous version of these with stock-style flanges that apparently worked fine, why they moved away from it is beyond me. They've even kept photos of the original ones bolted up on an actual car on their website, yet sell a 'revised' kit that doesn't work. Again to reiterate - these will not bolt up to the stock exhaust without extensive modification.
The position of the O2 sensor bungs is way off. Even cut down the Bank 2 O2 sensor is hitting the chassis, even with the heat shields bashed in. Could have clocked them differently since there's heaps of room to facilitate it but guess they never bothered test fitting. They could have omitted the secondary O2 bungs entirely since they're so tight against the chassis they're unusable anyway.
I don't believe the stock steering linkage would have cleared even with brand new engine mounts so be prepared to fork out for a slim one.
The supplied gaskets are garbage and went straight in the bin.
As far as RHD alternatives go, there doesn't appear to be much choice out there. There's Gravity Performance, who don't ship to NZ, and there's Supersprint that are eye-wateringly expensive. There's also Coby ones that look to be mild steel and would still need some welding to mate up to the rest of the M54 exhaust.
All in all, with the purchase price, paying for having these re-welded, the slim steering linkage and new gaskets, this exercise cost me around $1.2k, which is still pretty reasonable in my eyes, with @Eagle also taking a bit of a loss on the initial purchase. Despite not quite getting the sound gains I was hoping for, I feel it was still a worthwhile thing to do and a step in the right direction and I'd still do it again if I had to. Just would have been good to know what to expect before jumping into it so hopefully this might help someone else out there.