The after market oddly don't seem to do anything for the D3 4.2 petrol that i've found. I know they are far less reliable anyway.
There's an argument of getting a decent cat for elsewhere and welding it into place!
However i'm currently looking at getting the LPG working properly as that's something more useful in the long term and something I should have done a while back. I've just tried swapping the MAF (bosch) and will give the LPG software another play.
Initially getting Cylinder 8 missfires though!
There's a thread here (different car)
https://www.pistonheads.com/gassing/...&f=23&t=866609
Which mentions different gappings for gas which may be something to look at
And another which also mentions about the HT spark leads may be an issue with arcing due to the higher specs
https://www.pistonheads.com/gassing/...&f=69&t=906093
Which also may be something to look at