If you can get a Euro cam you will get 175 hp, instead of the US spec of 155.
I would say you can get quite a bit more. My 264 ideals very smoothly, so I am sure you can do longer duration. Duration usually assists mid to top end, so I also figure in some extra lift for the lower rpms, where the m103 is kinda weak.
A 274 duration cam is not uncommon for BMWs m20, SOHC 2.5 L straight six. Schrick also sells a 288 duration, I would say that 288 would be the top of the duration without a bumpy idle, but who says that would be bad?
Anyway, there is no reason to use a Euro cam, unless you just want a little. I am not really that impressed with the 264 (probably the euro cam specs, as it was a normal grind). If I had a second car I would go all the way to a 288 and see what that is like!