A few comments, Allanex50....
First, please don't use all caps for posting. It is akin to "shouting".
Second, if the aggregate size is correct in your mix design for the actual rebar spacing, a 150mm slump should be fine, provided appropriate consolidation measures are being used in the field (vibration). If you are vibrating and still getting voids in the concrete, then it is likely you have the wrong coarse aggregate size for the rebar clearance and you should do as JedClampett noted...use a smaller coarse aggregate. If you get the slump too high for a vertical placement such as this, you will risk segregation of the aggregate.
Third...no one here can give you "permission" to go to a 200mm slump. That will have to come from the structural engineer of record or the architect. Perhaps the design did not appropriately consider constructability, so you might need to work with the design team to get this right. I would not recommend arbitrary nor capricious changes.