With all due respect,
If you still have the old CASA issued, typewritten, A5 and woefully inadequete flight manual, it most certainly is still legally binding and you must therefore apply the 15% (or more if MTOW exceeds the set limits). I believe CASA conducted their own flight testing on the aircraft to arrive at their data.
If you have the AFM/POH from the manufacturer, odds are it will be miles ahead in terms of detail and if it had been approved by CASA (in terms of you have notified them of the change in flight manual and they issue the approval sheet that specifies the exact document and is to be inserted in the front of the new flight manual) then you are permitted to utilise the data contained within without factoring. Be aware if ramped they will frown on retaining the CASA manual in the aircraft if you have a foreign manual approved.
There is a note (couldn't be bothered quoting it) that where there is a significant discrepancy, the foreign manual data should be treated with caution. Basically saying use worst case scenario, but not illegal to use best case.
As already stated, an ops manual may indeed require a greater margin than the manufacturer specifies and as always, if you are operating under that AoC, you are legally bound by it.