Correct, the Helix catted DP removes 2 for 1. If you plan on having ANY more power, you MUST remove the precat in the uppipe, otherwise this is about as far as you can go. Yes, it is the most effecient cat in your car, but it is also the most dangerous to have if you up power anymore. The reason is that with too much exhaust flow, it will just blow the cat right out of the UP and obliterate your turbo. As far as I know, I have never heard of an aftermarket uppipe with a cat. If you want loopholes, my dealership says *officially* warranty will not be voided as long as it is a Subaru part, so you could put on an STi uppipe, but I think your concern was emissions or moral values; I'm not sure.
"We live for a good time, not a long time."
RIP Colin McRae