With respect guys -- If he keeps the stock cats, there is no way that an aftermarket X-pipe gives 7 HP over the stock H-pipe. All other things held equal, an X-pipe gives no HP over an H, there is only a sound difference. If the aftermarket pipe is truly less restrictive to any measurable degree (which I doubt), he might see 1/2 HP difference. No human on earth will feel that, and it's even within a reasonable margin of error on a dyno.
Now, getting rid of the cats, or going with the less restrictive high flow cats, changes that entirely.
My advice would be to get rid of the stock cats or don't bother. If you go cat-less, you'll pick up top end power but lose a slight amount of torque down low. You'll likely need to retune the car for the lack of cats. If you have a tune already, you might notice the loss of torque. If you don't have a tune now, the power increase from the new tune will mask the loss of torque when you remove the cats.