Audi A6 C6 2005 Avant Quattro 4.2 V8 (119000 miles)

I have an emissions warning light that comes on with a fault code:

16814 - Catalyst System; Bank 2 - P0430 - 002 - Efficiency Below Threshold

So far I have had a set of spark plugs, lambda sensors tested, new air mass meter, and still get the warning on after a couple of day's driving after fault reset by garage. The indications are that it is a problem with the catalytic converter (there are no rattles from the cats or blows in the exhaust).

I've had a chat with a catalytic convertor specialist who says not to replace if the emissions will pass an MOT. Apparently the car has 2 cats per side and is very costly to replace.

Just wanted a bit of advice as to what to do - i.e. what are the implications of having the warning light on. I have also heard that if I have an LPG conversion then the cats are not required to pass an emissions test because of the cleaner burn (although I have heard that it might be illegal to remove the cats). So for the cost of a new set of catalytic converters can I consider and LPG conversion and removal of the cats and see it as an offset saving on the conversion cost? Also of the 2 cats per side, one set are redundant as they are only required to enable the North American emission test to be met which is more stringent than our UK test, so I could have these removed anyway.

Any advice gratefully received!

Rob