As other said, if your trap is plugged, you're in for a shock when the car comes back from the dealer. If the trap is clear and just being removed as preventive maintenance, the increase is 5hp and 1-2mpg.
Mine was OK before but when removed, there was a noticeable power difference, and MPG increased too. I'd like to pull out the new catalyst (downstream in the new dealer-installed free exhaust) and see if there is a few more HP to be had. You just need to install the old front pipe, or ask the dealer to leave that one piece on the car (don't know if they will, though.)
BTW - easy power test is to clock the car 0-60 on flat road, should be 11 seconds (at sea level). If much slower, say over 13 seconds, something isn't right.
Good luck!
