Replacing stock XK8 exhaust manifolds with headers

[Eurowest-XJS]

Hi,
Does anyone know of any company manufacturing a set of performance exhaust headers to replace the restrictive stock exhaust manifolds used on the Jaguar XK8. The only reference I find when I do a search on the internet, is that Borla company made a set of prototype headers for the car. And also, does anyone know if the stock exhaust headers used on the Aston Martin V8 Vantage will fit the Jaguar XK8 chassis. A picture of these can be seen at;

http://www.motortrend.com/roadtests/.../photo_08.html

The factory part numbers for these Aston Martin V8 Vantage exhaust headers are 6G33-5G236-BC for the right side and 6G33-5G232-BB for the left side and the factory prices are $1616.49 and $1432.55

I am thinking that because the peak horsepower rating for the XK8 is listed as 290hp at 6150rpm, and the car does not accelerate very strongly past about 5200 rpm in actual driving,even though it will rev to redline with the shifter button in sport mode, I am thinking that the restrictive exhaust manifolds are what are really holding these Jaguar XK8's back.

I will greatly appreciate any information that anyone has on this topic.

Thankyou,
James Denno