You will get minimal gains with just an exhaust, claimed gains are utter horses @*!#... and definitely no gain if you don't remap. It's your cash, but you could buy a cobra cat back and get pretty much the same sound... and spend the £2000 you saved on something else. Unless it's Ti then a £2700 stainless exhaust is about £1700 overpriced.
Like I said, it's your cash, but you'd do well to listen to other members on here. Between us all we've seen and heard several hundred different Zeds in the flesh, so we do know what we're talking about... Most of the time anyway!