There are at least 10 dynos showing catbacks on stock SVTs gaining 5-10 (average 7) FWHP. The resonator with its narrow down to under 2" is likely the biggest single bottleneck of the stock SVT catback so Demon's 3FWHP estimate is pretty much what you can expect replacing the resonator with straight through part. Not much but then again its only about $80 and sounds better.
I've seen a number of dynos on a number of cars..they rarely show more than 2-5 HP with main cat removal.
