For the US model, 79 and carbed 80 were probably the lowest hp spec since they had a terrible carb. Same deal with the 124 spiders....the standard fix for the 124 is to get a carb and intake manifold from an earlier spider and throw out about 30 pounds of vacuum hoses and various emissions things. I have no direct experience with the same model Xs but assume the "fix" is the same.
From what you guys say, the Euro Xs had a nice exhaust manifold, higher compression, better cam and a decent carb so of course they had a higher output than any US spec X.