Honest Answer - for the adapter's you have described as used in your landscape, PO 7.5 provides nothing new.. What it does though is provides a host of other features which if relevant to your client might add value for which you need to go through those blogs and understand if any of them can add value..
In my opinion, the main reason I like PO 7.5 is the support for cloud integration content, which means standard HCI Content can be deployed on to a PO 7.5 server and used as-is. PO 7.5 has a partial runtime of HCI which means HCI Integration Content can be leveraged on PO 7.5 enabling customers to use standard pre-delivered content from SAP. Is this relevant to your customer., well only you can say!
Regards,
Bhavesh