Hi Luis,
You can use Chunk mode as mentioned in PI/XI: PI 7.3 processing of large files - teaser if you don't need any transformation. Normally 50 mb XML file size should not be an issue for PI.
RFC does not have good performance for multiple calls. Try to use with Maximum Connections not more than value 10 at Receiver RFC Channel and check how it behaves. It would be better if you could use Proxy at Receiver end.
Regards,
Nabendu.