Don't get pattern, they are very, very poor. Sound awful and won't last, you are better off getting second hand genuine of a breaker here
Typo fixed.
There are several threads re pattern cats being substandard. Not surprising when you consider the cost and build quality. Pre cats on the Dbw cars aside as that's a different issue and yours dosnt have them anyway, the gm v6 cats are pretty bullet proof, unless there's been a fuelling issue to damage them.
I wouldn't hesitate to fit the used gm cats on an omega, as a rule. Cheapest and most reliable option given the alternatives. Granted there's an element of risk the used ones might be damaged, but you could buy several sets for the cost of gm ones, and pattern ones are shite by all accounts so a complete waste of money, going by posts here.
No personal experience of pattern cats though, must be said.