Err.. makes a MASSIVE difference USB 2 or 3 on the connector/adapters.
I import RAW files to my IPP using the USB connector (or SD reader) I connect the USB connector to a multi card reader, and the other end to a power supply. The files import blazingly fast this way. MUCH faster than USB2.
The only reason I'd go for 256Gb is to have the iPhone be a portable backup in the field for my files. Now that Lightroom Mobile natively supports loads of RAW files it's pretty seamless when syncing back to my desktop. It works REALLY REALLY well and I can sit and rate/cull/edit on my IPP and I wanted too do this on my phone too.
IIRC iOS10 has more of an open structure for handling RAW files too.
So yes.. for importing of RAW files the USB 3 on the IPP 13 inch is a huge boon.