why would you NEED a high flow cat? everything I've read about exhaust has stated replacing or removeing the cat is an overrated modification, often unnecessary. You usually don't see much benefit from doing it.
You could just have your own flex pipe welded with an excort flange pulled from ajunk yard or off eBay (as you stated), shouldn't cost more than $70 tops for a weld job of this type (rather than doing a custom type one). I doubt our flex pipe could be easily replaced with a 'universal' one anyway...
I was estimating $550-$650 WITH a cat-back exhaust.
2002 [Black] Focus SE :SPI:
2002 [Sangria] Focus SE :Zetec: