I've got an ex Mistral racing 1216cc side car motor in my slabbie.
The cases have a huge amount of work to help pumping losses. The bottom if the liners have big cut outs to allow flow between cylinders and the webs have been de bured corners radiused. I've also added a big -10 breather to the clutch casing and opened up the cam cover breather. They both go into a large catch tank/ seat support. which is vented to atmosphere via a big air filter.
I've seen healthy gains on race car engines from a good breather set-up and one engine that lost 25bhp,around 10% ,because of a partially blocked breather.