So myself and a few friends tried the 15-20 camp spot, and we didnt see a whole lot of Yarzons... in fact we only saw one lol. In the end we decided to try Cassiopeia out and we SP'd on the cassiopeias r25-29 that spawn near the beginning bit (the spawn area of the Battlewarden). There were 5 of us and when we began, the range was 13-16 (I was r15) and I personally was getting 333sp per Cassiopeia, and there wasnt much downtime so I think that should be added as a camp, provided there arent more than 2 parties there at a time.

EDIT: I think its feasible to SP on them till rank 20. Im now r17 and still getting 300sp per mob.