Hello Ashish,
If I understand your question clearly - As this is a transformation case and as you have mentioned that RFC is already created in SAP, I would suggest to go ahead with Server proxy and calling RFC from within.
Advantages:
1. Reuse of already tested RFC.
2. Considerable reduction in development time and effort (Considering there are many such transformation cases as per your description)
3. Better performance.
4. Data format validation can be moved from PI to ECC default proxy calling/handling depending on your scenario.
Regards,
Ankit
