The 1.8 head has been know to be a popular "upgrade" for a lot of 16V guys. Here are the real differences. Though the 1.8 head can help with some top end gains, it is limited in torque. This is due to the port design. The 1.8 head has larger intake ports, but smaller exhaust ports compared to the 2.0 head. The overall power band can suffer depending on the displacement and cam combination. So, the 1.8 head is not always the better way to go for every application. The 2.0 head has a broader power band and more available torque.
To examine this more closely, you need to consider air velocity (the speed at which the air travels through the head). There must be a careful balance between intake and exhaust. The induced air (consumed volume) must be able to exhaust (escape) from the head at a rapid enough rate in order for more air (from the intake) to be converted into power efficiently. Unfortunately, the efficiency ratio is disproportionate for all 16V heads, generally. The goal of porting and polishing is to try to equalize the efficiency of the ports. Either head (1.8 or 2.0) will have the same result if ported correctly. So to make a long story short, it is important to consider where you want to dial in your power band, and cams are also a huge part of that equation.
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