Is there a replacement that fits the stock header and exhaust, that I could add a high flow cat?

 

is there a specific reason you can't just put a high flow cat in your stock pipe? not that it will make any difference.

 

Maybe I've got this wrong, but I thought that opening up that location in the exhaust would provide a bit more flow.

 

yeah, if your current catalytic converter is clogged, then it's make more power, cause you'd be making less power as a result of a clogged converter...but just going from an OE cat to a high flow won't improve performance unless you've done something to the engine that requires better flow.