According to the late Bill Byldenstein, there is nothing wrong with the standard downpipes and cast manifolds on the VR6...so I wouldn't go expecting any gains on a standard engine.
Certainly the 225+ BHP 2.9 VR6 I built was managing just fine at 7300 RPM (with a big valve head and Schrick 268 cams) on standard manifolds and downpipes.
Personally, on a standard engine I'd spend the £300+ elsewhere...unless you absolutely NEED the bling.