it will definitely help performance. 20hp i don't know.
you could also modify the profile of the cone in your existing pump to achieve the same results.
dr. diesel or andy2 might be able to shed more light on this... i haven't done this mod myself (yet).
edit: if someone could post a picture of a stock cone, you could compare them side-by-side to see where you need to remove material to achieve the profile of the 20hp ebay cone.
Print that photo. Then, use the picture as a template, when you cut/machine your existing control cone. It is very easy to do, just remember to highly polish where the pin makes contact with the cone.
Here's the picture of a stock 1.6TD boost pin.
Hope this help..
Marc/