A couple of months ago in Fast Ford Mag one of the contributers did some flow tests with his new ported head. The final test he did was to replace his swedish plenum with a stock 4x4 one. Flow through the valve was reduced by 24%!

In my book thats a pretty good reason for replacing the stock one at any power level.

Remember the stock YB was built down to 200 bhp on ford request so cosworth weren't too fussy when they did the final design for the manifold.