In his book "Four Stroke Performance Tuning", A Graham Bell showed some very interesting things regarding branch manifolds. In short, aftermarket manifolds rarely give worthwile improvements over stock manifolds. In the lower rev ranges the stock items are usually superior and the aftermarket may typically only give an increase of say 2 bhp @ 7000 rpm. Is it worth the money then? Alfa manifolds have long been known for their good design in stock form, and the 155 is true to form as well.
Where aftermarket stuff will be superior is on heavily modified engine where high engine speeds will benefit from pulse tuning. But on our standard engines you will benefit more by just dumping the cat and replacing the rest with a larger bore system.
One can also go and match the manifold to the exhaust ports, to make sure there are no "steps" into the manifold.
If I remember correctly, a 4 into 1 manifold will give better top end power, but inferior low end power. A 4 into 2 into 1 (like our cars have) is more balanced between top end and low end power. Long primary pipes are good: They lower the range where maximum is produced.
Hope this is of some help.