Will removing /system apps give any usable space on lollipop? In earlier versions I've pushed updated apk's to /system manually from time to time as it annoyed me to have an extra older version of Google+, Play store, Youtube, Keep, actually ALL the preinstalled google apps as well as a lot of Sony Apps (Album, Music etc.)
This would give me 300-500Mb for a cleanup as some of these apk's are quite large.

This is a tedious approach and would break stuff from time to time, but I am unsure if the simpler approach: Merely deleting the old system apk would give me any useable space?
In older versions the clear answer would be "no" as each partition was allocated a set amount of space, but after (I think) Kitkat, Internal storage was mounted as an emulated partition or something like that.

I have been searching for more information regarding this, but have not found anything yet. Hope there is a knowledgeable one hidden among you (preferably with sources) ;)
Nope. Deleting things in the system partition has no effect on the amount of usable internal storage - the system partition is reserved for the ROM and mounted read only by default. Only deleting things in /data and /sdcard will affect the amount of space available to you.