Look at it this way, what benefit is it to them to limit their target audience by saying it requires mods that they don't sell? They obviously feel the stock downpipes are too restrictive for that level of boost.
It may feel great with the increased boost, but unless you're logging (and I don't even know what you would log for this, temperatures?) you could be doing damage. Obviously no one will recommend doing something that increases risk, even if you say it's just for a little bit - let's face it, how often will you really switch back after you get used to the Stage 2 power? Gotta pay to play.