Is this a straight 6 or the V8? I have had something not dissimilar on my V8 which apparently now (after 2.5 years and many thousands of £££) is fixed. As I found out to my cost, there are many, many things that can cause these symptoms and the diagnostics don't help a great deal, in fact they can be utterly misleading. It's a pity you're not nearer becuase (if yours is a V8) a have a garage loft full of parts that were changed but weren't the cause of the problems!
I tend to agree with you that it's not likely to be the cats, but to be honest I don't know what the effects are of taking them off so I can't comment. My car is completely standard M62.
Let us know how you're getting on and I'll contribute what I can to the discussion.