Check out HJS cats, made in Germany and apparently very good quality. I just bought two 200 cell cats for around £640 delivered. I put a pair of HJS cats on my previous S8 too, I had no problems for 4 years before I sold it.
The only catch with 200 cell cats is they may trigger the CEL from the post-cat lambdas. A simple solution is either spark plug anti-foulers to move the lamba out of the exhaust flow, or to get the post-cat lambda error coded out. The lambda has to stay in there as it is still used in the fuel maps (as far as I understand it) but it wont produce error codes or trigger CEL.
On my previous S8 I had a pair of these Wayne Angles to stop the CEL, and I only had a intermittent post-cat lambda error code but never a CEL.
http://forums.vwvortex.com/showthrea...ear-02-Cel-Fix)
http://www.hjs-tuning.de/de/3/71/0/s...universal.html
I bought a couple of these HJS-Nr. 90 95 0092
I got them from here, good service arrived in less than 1 week.
https://www.turboperformance-shop.de...s-bis-2600-ccm
They should be fine for our engine size and HP looking at the HJS data. Or you can get bigger ones if you want.
Also I'm not sure if you may need a remap because of freeing up exhaust flow? If your current map is to get the best performance with current cats, you may lose some performance with free flow cats unless mapped for them? I cant really advise on this we need a engine/exhaust guru to answer that one.
More info from AWE who also sell HJS cats. Cheaply made aftermarket cats can telescope because they are wound just like toilet paper. Properly wound cats are wound in several ying-yang spirals (for want of a better description
) for a much stronger structure.
http://forums.vwvortex.com/showthrea...t-vs-metal-cat
one more bit of info, MRC charged £150+VAT to code-out post cat lambda errors. However, if you are going for a completely custom map maybe you wont need the post-cat lambdas at all?