I can see that in Europe, fuel with nitro is expensive, so you have to find the way to obtain good power with less nitro.
I had not idea about adding shims to a Webra engine but I can see the reason. Been an european engine should be fixed to run fine with less nitro. What I remember about the webra .40 sized (or so) that I had about 15 years ago is that it was a really reliable engine and very powerful.