I hear a lot of people saying that Corsas are the only stingers that don't have any drone. I dunno about as I have the Steedas which drone a little bit at certain rpm's but if that's what you're worrying about you could probably save some money in the long run by getting another brand of stingers and adding an X-pipe which is said to eliminate drone in these types of exhaust. This of course is for the 4.6L engine. I can't say what it would be like for the 5.4L but I don't see how it would be too different.
If your talking about a GT500, yes, it already has an x-pipe. Your in the wrong section, as this is for GT's not GT500's. But thats ok. From what I have heard Corsa's are a pretty penny but worth the money.
If you already have an X-pipe then you really shouldn't worry about drone that much so I'd go for a cheaper option if you want to try and save some money. Pretty much all other stingers are made by Borla including Steeda and Ford. The tips change which alters the tone just slightly (but not much).
I started off with Corsas but they weren't really loud enough and damn, they're heavy! I changed mine out for a custom-made system and you know what? No real drone either, just a little more volume (maybe not enough...). http://www.cardomain.com/ride/2177534/6